Permadeath + Endless on unit. Unit will not Purge.

[MATEJZAO]

I just started a run with Evil Eyes and when my units die, they get purged. However, I upgraded my imp with Endless which died multiple times in the same battle. It was alive at the end of battle and now I still got it in my deck.

The next battle all my units died and boss got to the pyre, won the battle and the imp is still in the deck. So even when all units die with endless, they are not purged. This is probably the most broken upgrade for purged units.

[Loviatar]

Well, you will find there are often ways to 'break the rules' in the game, and that it's something that is relatively encouraged to do. I think that helps explain the appeal of the game, in that you can sometimes work some crazy combo that helps you survive the entire run.

For the Endless thing itself, I think it's also in how the game processes cards. You will notice when you play a unit card, the card 'disappears'. It is now effectively in play on the 'board' of the train - imagine playing a physical card game, you would place the card down on the table, whereas in the game, they get turned into model representations.

Normally, when a unit it killed, you will find it's card in the Consumed pile, which is where one-per-combat cards go. However, with Endless, the card immediately goes back onto the top of your draw pile. Purge just modifies the Consume trait to remove it from the deck once consumed. Since your Endless unit never goes to the Consumed pile to get 'consumed', it can never be purged.

[Limdood]

It makes sense....you have two conflicting rules that trigger on the same event.

"Whenever a non-champion unit dies, it is purged"
and
"Whenever this unit dies or is eaten, it is put on the top of your draw pile"

The game could be following the "more recent rule takes precedence" (game long effect vs. temporary card - I'd be interested to know what happens if you put a monster railspike on a naturally endless unit), or the "more specific rule takes precedence" (This unit vs. all units), both of which are very common for games that have situations where two rules can conflict directly with each other.

Endless has some other quirks that set it apart too....If you use the Umbra or awoken tome to apply Trample or Quick to a unit, and that unit dies and comes back (through having endless from another source, though it works the same if reformed), it will no longer have Trample or Quick....but if you apply Endless to a unit using the Melted tome, and it dies, It will still have endless the next time you play it. Despite both Trample/quick and endless being non-stacking "yellow" traits, both applied by cards instead of being permanently given, and both being on units that die while having endless.

Monster train does a fairly good job of keeping rules consistent, and giving you the information up front (you generally know exactly what will happen during combat when you hit "end turn," with very few exceptions) - but every now and again you find a quirk that acts weird that you can't really guess what will happen and just have to see.

[Gabriot]

On a somewhat related note - if you get the monter railspike event, where the purge of the unit occurs on summon - you can actually keep the unit if you have endless and allow it to die before the battle ends, but don't replay the unit. It'll only stay purged if you play the unit and it is alive by battle end.