The Tavern

This monster currently has two spellbooks. One contains bolt of draining/stone arrow/haste and the other contains iron shot/pain/haste. These books are functionally very similar, without even the conceit of, say, inflicting a different status on the player or checking different elemental resistances (as is typical for monsters with multiple books). Additionally, pain deals d15 damage if it beats magic resistance and the player doesn't resist negative energy. That is far too weak for a monster that is supposed to be threatening in Crypt. I suggest giving all ancient champions iron shot/bolt of draining/haste.

I think it'd be good to take a broader look at monsters with multiple books as well; there's significant cognitive overhead in dealing with a monster that has a subset of e.g. twenty spells, and a player may need to examine one monster multiple times (because they gain new information when it casts a spell, or because the monster appears with others of the same type and some of them blink, for example). The use of multiple books needs to add sufficient gameplay depth to justify that cost, and I suspect that many of the monsters that use multiple books do not do that. Such monsters could be reworked to use a single book or split into multiple monsters.

Unless the splitting is "create or better rework" some early monster to introduce sooner some mechanic only present in late game, please don't do that.
There are already too many monsters with weird gimmicks that actually don't provide nothing too interesting.

Send us some PRs you coward we know you know how to write them.

The ancient champion change is almost as easy to make as it is to post about it. The broader project would be a lot of hours of negotiation for someone without commit access. Just my two cents.