Crystal balls can be used to find which level any of the unique items is currently on. Unspecified how it would show different branches or if this works on items you have never seen (if it did, you could use it to look for the Amulet and figure out how deep the dungeon is).

A special container that can hold the invocation items, and only the invocation items. Has bag-of-holding-like properties (or else is an artifact bag of holding). It appears in some hard to reach place in the dungeon.

Add a unique lich or demon lord enemy who may appear as a side effect of raising the dead with the Book of the Dead. Not specified if this is the only way this monster can ever appear, or whether other mechanics like demon gating could bring them in as well.

If you chat to the Oracle while carrying invocation artifacts (or possibly just after you’ve triggered the game flags for having acquired them at some point) she offers you a special consultation which helps show an unspoiled player the next step to take to complete the game, given what you have so far.

Priests get buffed effects from the invocation artifacts:

• The Bell of Opening opens things at range.
• The Candelabrum’s light is brighter (or “sunlight”) and affects things sensitive to light such as gremlins and vampires. The candles could also burn slower.
• The Book of the Dead either improves the power of turning undead, or makes it easier to raise the dead.

The cursed Book of the Dead will occasionally raise the dead around its location on its own without warning.