Originally posted on my World of Dragons pack issues tracker of which I am the developer.

AbyssalCraft's Staff of rending and its subsequent upgrades does not harvest energy from abyssal mobs even tho it damages them and kill them.

to collect energy from the staff you right click with the staff in your hand and the crosshair on the abyssal mob

Minecraft: 1.12.2
Forge: 14.23.5.2806
Mod: 1.12.2-1.9.4.14

reference: https://github.com/kreezxil/World-of-Dragons/issues/33

You need to be in the Abyssal Wasteland for it to work

This could potentially be caused by a change made in Forge 14.23.5.2805:

Add a hook to allow continuously using items through stack changes

It's either that or something else in the modpack. I'm gonna run some tests and get back when I know more.

EDIT: Staff still works in 14.23.5.2805. This is caused by something else, not AbyssalCraft.

The problem was caused by scaling health and forge creating an issue in which weapons were in a state that didn't loose any durability, didn't get exp ( tinkers' construct ) , didn't get energy (abyssal staff) and it also caused the ender dragon and the spectral dragon becoming invulnerable.

Forge build 2807
ScalingHealth-1.12.2-1.3.32-134