Spiritcaller Backport is exactly what its name implies; Basically we're talking about backporting the Spiritcaller from Illage and Spillage to 1.16.5, here.
Note that this mod will not work on servers. This is a bug, but 1.16 as a modding version has been long abandoned and thus it is very unlikely this will ever be fixed.
This mod does come with its own config options to use for Spiritcaller, just like in Illage and Spillage. Although the only point in backporting Spiritcaller was for 1.16 mob battles, the Spellbound Book has also been backported to 1.16.5.
This mod does not backport any features in Illage and Spillage other than Spiritcaller. Spiritcaller was a tedious pain to backport; I as such will not be backporting any other features.
Boss Randomizer
If you want to add Spiritcaller to your 1.16 boss randomizer, type this into Boss Randomizer's config list (the I&S config file):
"spiritcaller_backport:spiritcaller" (quotation marks included)