The Aviontix believed in the eventual reincarnation of the body. It was mummified in a special way to make sure that after it was interned, the spirit bird of the individual - his soul, would fly from this realm. If the spirit bird returned, it might need of its body, so the dried remains had to be preserved. This is all interpretation, as the Aviontix fell to sickness and civil war long before any other intelligent beings walked their lands. So while they have lefts us books and murals with an elaborate writing, very little is known about what they do think.
One Elven source has this prophecy attributed to the Aviontix: "In the days of the final dawn, all good people would come to roost in the lands, to live in a paradise reborn and defend it from the last night." It stands to reason it was the duty of all good people to leave bodies behind so they would eventually live in the paradise reborn. Of course, they would need things when they returned, so they would be buried with money, tools, clothing, and comfort items.
So everyone has a large tomb. The business of death would have been a big one in this land.
An Aviontix tomb will be a large affair. Lesser peoples will have family tombs, while nobles will have huge building sized tombs. It will be made of large stone blocks, crafted with such care that a piece of parchment does not fit between them. How these blocks were transported from quarries so far away is a mystery (magic?).
The entries will be blocked off of course, to prevent robberies. Those with time and tools could eventually dig their way through the rocks. Thus traps were employed. Later cultures who have records of the various Aviontix tombs and ruins they explored (plundered), denoted hundreds of cunning traps that will be triggered entering into the tomb, but will not activate upon exit (usually with the pull of a lever or two in the main crypt). Crushing traps (that eventually reset), pit traps, flying spike traps, dart traps, flesh scarab traps, animated statues, tiny alchemically created defenders, elemental bird spirits that attack after a sconce is lit, and hundreds more.
The difference between Aviontix traps and most other peoples traps is that they were defeatable by solving a puzzle. Some of these puzzles were word/ poetry puzzles (which are undecipherable now), number puzzles, image puzzles, shapes in a pattern, and even musical puzzles. Things scattered in the tomb had to be retrieved to deactivate the trap (which would be set off if you pass a certain area) were common. (Taking the blue hexagonal tile and replacing the red one, and red to blue would deactivate a trap), or tiles had to be moved about (in sliding tracks or spun in place), or organs played. It was a battle of wits between the designer and the thief. The Aviontix must have admired cunning over strength or dexterity, because solving the puzzle gives you the prize. (Another competing theory is that people/ priests might have to enter tombs to do things for the dead, so the temple might of had the combinations to each tomb.)
After a gauntlet of traps, there will be either an apartment (where the risen might live for a while) or a storage area (with materials that might still be good after all this time). Then there will be another gauntlet of traps, and either an apartment or a crypt for pets or servants. There would be another gauntlet of traps, and the true crypt.
Larger noble tombs will have a variety of branches to confuse intruders, all with traps and mock storage and lesser crypts. These false branches will lead to traps of ugly death (those Grimetooth would be proud of), as anyone who was supposed to be there, would not of taken the odd branch.
All the best things will be entombed in a sarcophagus with the dead. Opening the final trap of the sarcophagus is normally a life and death proposition, so great care should be taken.
(Many crypts have the main crypt hidden behind a false crypt. So people will loot the false crypt, and leave the real crypt alone).
Nobles will often have undead servants with them, to take care of interlopers. In fact, many spirit birds return when crypts are disturbed, animating a very peeved off undead creature of great wrathful power when the sarcophagus is opened.
Now most Aviontix tombs have been long looted. Yet, there is always hope that somewhere one will be left undisturbed with its great wealth and powerful magic items intact.
Remember those that die in these tombs should go to Merideth, The City of the Dead. There they will wait for a time before being "called" by their Deity, if their deity is inclined. If applicable and the GM wants to run it, if the dead are to be resurrected, their soul must be rescued from Merideth first.