Who is dark, the god or the religion and society?

You wrote that the priests sacrifice slaves. Well, that is evil. To make it less evil, have them sacrifice volunteers. One cannot expect informed consent by 21st century standards, but make them think that the sacrifice is necessary for the prosperity of the community.

• No sacrifice, no crops. "In the first and only year of the reign of John Doe XIII, there was no sacrifice. A hailstorm killed the summer harvest and the fish did not migrate upriver. The next year John Doe XIII was the sacrifice and all went right. We're telling that story to would-be reformers ever since."
  


• No sacrifice, no family. "In the high nobility, the firstborn son goes to The Tayan. That ensures that the second son has a long life, and that the daughters will bear many sons. The 'right to ensure continuity' is a jealously guarded privilege of certain noble families."
  


• Sacrifice as penance. "That was a mortal sin, my son. 1,000 years in purgatory after your death, or 5 years in The Tayan before your death. Your call. Here is the waiting list."
  

You can make flawed societies sympathetic. In fact, you pretty much have to, unless you're writing about a utopia or writing an anarchist manifesto.

In many fantasy and historical stories, the good side has public executions and mutilations, torture, slavery, etc. And yet, we're able to see them as the good side. Sometimes, the other side is more evil; other times, there's an Evil Grand Vizier who's worse than the society as a whole. Meanwhile, we meet dozens of people who help our heroes, who are brave and kind, and painted as sympathetic even though they don't know anything better than the society they grew up in. Maybe you'll have a scene where one of the wiser good guys laments that they can't end slavery because stopping the Evil High Priest is too important to focus on a quixotic crusade to overturn a fundamental institution of their culture.

In fact, you can see all of that in the Star Wars movies—not to mention the Ewoks, who kill and eat other sentient beings—and yet nobody comes out of those movies saying, "What was the point of all that when the good guys are evil too?"

For a more directly relevant example, see the 1964 Doctor Who story The Aztecs. Aztec society is painted as in many ways better than European society at the time (and one of the Doctor's companions, Barbara, directly points this out in case you miss it), with human sacrifice as the one thing that makes it worse. Most of the good guys (not counting our outsider heroes) continue to believe the religion they grew up with, but don't come across any less sympathetic. The one exception, we're pointedly told that he's not going to succeed in changing his society, and see him as a tragic hero.

You call it a dark god, but that's you imposing your culture on the Dalians, to the Dalians it's just God.

Unfortunately it seems you've been brainwashed into thinking that human sacrifice is evil. Sacrificing the occasional person is a natural part of the cycle of life and death. People are born and people die, and all things in their time.

Perhaps your fate is to die in your bed of old age, but this slave was born to be sacrificed to the Tayan such that we may live and the crops will grow. That is his fate as decreed by the gods, and who are we to challenge fate.

The ceremony is one you might recognise until the climactic moment, the priests lead the way, swinging incense, the sacrifices dressed in white follow behind escorted by acolytes, they look like they might have been drugged but they're apparently willingly walking with the group. There's a short ceremony by the mouth of the Tayan after which each sacrifice is blessed by the high priest before stepping off into the maw. Another short ceremony follows asking the Tayan to accept the offering and bring good harvests for the coming year, along with any specific requests that those who donated slaves might have.

As secondary note on this, the Dalians wouldn't know how long it takes for a Tayan to digest its dinner unless someone had escaped. Once the sacrifice is out of sight, it's as good as dead.

Its really all dependent on what they believe in and not what actually happens. Often a correlation between two things can doesn't mean they are really linked.

My proposal is that they believe that the offerings bring them good luck and blessings from their God. Its a pretty simple setup.

One day someone fell in, and that year the harvest was really good. This happens a couple of times with people falling in and good harvests. Then one year no one falls in. They are super careful, fence off the area and make sure no one falls in. The harvest is terrible and people get riled up and angry. They sacrifice someone to the Tayan and the next year the harvest is good again. After long enough many people will simply believe that having a sacrifice for the Tayan is what brings in a good harvest.

Now there doesn't need to be a strong correlation, just that the people believe its there. So maybe one year there is a sacrifice but a bad harvest. Well some local person goes, the Tayan isn't happy with the sacrifice, we need to sacrifice more people. Once enough people believe there is a correlation, they will likely carry it out regardless of opposition. (Maybe the Tayan is upset that your doubting its will, sacrifice him and his family!!)

Now the fun part for you is to decide the different factors. Maybe the Tayan likes Female Virgins, or male slaves. Maybe its about purity or strength or quality. They could also believe that the longer a person suffers for, the better the harvest and choose an unlucky person to suffer. You can spin it to be a great honor (with the top families preparing special children just to be a sacrifice, or maybe they know the truth and shield their family from it but encourage the populas).

Make the Tayan important for their survival.

For example, the Tayan relies on prey to fall in, so it needs something to attract prey. It could do this by encouraging plants that produce fruit, nuts and other edible things to grow next to it.

In comes the mycorrhizal network: https://en.m.wikipedia.org/wiki/Mycorrhizal_network

This is a network of fungi that connect plants and trees, allowing them to share nutrients, send chemical messages such as if certain insects are eating it causing other plants to produce chemicals against such insects or even "hack" the system and killing off plants that are unfavorable, such as plants that would grow over others to take away sunlight. This is the system a Tayan could use to encourage edible things to grow next to it, attracting animals that might fall in the Tayan's mouth.

This wont go unnoticed by the society living there. The best and most (or only!!) food grows near a Tayan, and killing it disturbs the balance and the food disappears! If you block off the mouth of the Tayan it'll starve so... You are going to have to sacrifice something to keep it alive.

Tayan's would be revered as a dark God that provides you with what you need if you give a large sacrifice. It provides food for dozens more people than would normally be able to live in the area, and a sacrifice is going to be a small price to pay for all the extra people living there just above the Tayan's hungry mouth.

"Mother, why do we sacrifice prisoners and slaves to the Tayan?"

"Well, child, before we started to sacrifice, the Tayan would become angry, and come into the village. In his anger, he'd kill our livestock, destroy our crops and eat our children. No one was safe. Children disappeared from their beds.
Whole herds were killed in the fields. Families starved. The last time he entered the village, our chief, most noble Mandu, offered himself to the beast to keep it from eating a child. He took Mandu and went away satisfied. Since that day, we followed Mandu's example and offer up those who deserve death for their crimes. When criminals aren't available, the noblest of us offer their own property, their slaves bought with their own money, to assuage the anger of the Tayan. "

Maybe in their culture, they find a meaning in the death of some of their people.

Being selected could be seen as a favor and people wouldn't go against their fate. Maybe it's a random process, so every individual live their life as each day could be their last one, your tayan could represent "Mother nature" and its impartial selection.

My favorite is that somehow they (or the leader) know that their food supply won't stand so they need a way that every one tolerate murder for the many.

Note: I am changing some of the tenets of the question here, take the points that you like.

Limited, Voluntary, Punishment, "Eugenics", Meaningful, High Ranking, Merciful

• Limited You do not specify numbers, but those are important. A single sacrifice each decade is easier to forgive than a sacrifice each week. Also, the reduced numbers make it more likely to believe the following points:




• Meaningful It is clear that the sacrifices are necessary; if they do not happen then very bad things happen to the tribe. By being a sacrifice, the person sacrificed helps his tribe and family.




• Voluntary In today's individualism, the idea of killing someone against his/her will is very difficult to be received simpathetically. But still people who voluntarily risk their lives to save others are celebrated as heroes. You could try to change your system so that the sacrifice is a big honour to the victim (whose name and life is, for example, recorded in the Great Book of Deads, and his familiy is cared for) instead of chosing someone against his will only because he is defenseless. Better combined with "Meaningful".




• Punishment If you want to keep the sacrifice mandatory, at least try to limit its randomness; the victims are not chosen just because they are slaves who cannot defend themselves, but because they are criminals and they have been judged to deserve death. Each year, of all of the criminals sentenced to death (or life in jail), the worst are selected to be send to the Tayan. Of course, that means that you need to have enough criminals for the sacrifices, but if the frequency of sacrifices is low enough maybe that is not an issue¹.




• "Eugenics" Another way to reduce randomness; only people ill and/or very old are allowed to be sacrificed. Works better with "Voluntary" and "Meaningful".



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  High Ranking To the "Voluntary/Meaningful/Punishment/Eugenics" points a cynical could say that the populace can be manipulated by the powerful so that the sacrifices are always slaves, poor people and the like that have been "brainwashed" by the nobles and priests who do not desire to become sacrifices themselves. You can challenge that by making the sacrifices from the most influential classes.

  
  
  

  Make it an opt-in; if you are the son of a noble, when he dies you can either go to live as a commoner or inherit the title and state, but in the later case you know you might eventually become the Tagan's breakfast. Similarly, anyone opting for priesthoods will know that the end of the career is perhaps your god's estomach. Works better with either "Eugenics" (give people time to enjoy their careers) and "Voluntary", or with "Punishment".

  
  



• Merciful The part about the Tagan eating the sacrifice alive and keeping it that way in the estomach seens unnecessarily cruel. Execute the sacrifices as painlessly as it is possible before presenting them to the Tagan. If the Tagan requires living sacrifices, sedate them. If the Tagan requires living, struggling sacrifices, give them some drugs so they can take them after they have been swallowed.

Some men with otherwise modern sensibilities would argue Jephthah, the man in the Bible who sacrificed his own daughter to God is a heroic (or at least sympathetic) figure. Heck, the letter to the Hebrews in the New Testament mark him as a great servant of God.

The reason why is he made a vow to God; he'd sacrifice the first thing he saw when he returned home from war, a war that God apparently blessed him with victory on. Stupidly specific deal terms aside, he made a vow to God, and he followed through with it.

Maybe this society believes that they have a binding vow with this God for their continued prosperity or survival, then they'd not only sacrifice people to them; they'd consider people who sacrifice others to their God.

Cure Terminal Diseases/Injuries

It is also be able to cure diseases and injuries while it's at "digesting". If you're lucky, and enough other people get eaten you can look forward to being passed after your malady has been cured by the painful digestive process.

It's worth noting that 'god' doesn't digest it's 'victims' physically, rather it feeds off of the psychic energy of their pain and agony. In fact, it provides sustenance to keep them alive, which is what provides the ability to cure, however the feeding/repair process itself is what is terribly painful.

Honor and pride

A big dark creature eating people and needing sacrifices? Hey, it's the plot of Tim Schafer's Broken Age

Vella Tartine is a young woman living in the Badlands, a land ravaged by the Mogs, giant creatures that come from beyond a wall called the Plague Dam. They are appeased by the sacrifice of young girls at a number of Maiden's Feasts held at various villages. They appear every 14 years for those maidens. Vella is chosen to be sacrificed to Mog Chothra at the feast in her hometown of Sugar Bunting (formerly a town made up of warriors named Steel Bunting) but concludes that if the monster could be killed, the rituals could be ended.

What is interesting is that the sacrifice is festive! Maidens are dressed to be the most appetising for the creature, so families could be proud of having their daughter being eaten by Mog Chothra.

Watch this video from the lost world on a man eating plant. And they sacrifice to it to keep the village safe from the poison gas that protects them.

https://www.dailymotion.com/video/x287ds4