So a little while ago i made this artifact level magic item and i wanted to know if i made it to underpowered for an item of it's level.

https://www.dndbeyond.com/magic-items/1688555-staff-of-eternal-undead

I know it's probably way too powerfull to give to players but it was mostly designed for use by Big Bad Evil Guys to explain why NPCs can command hordes of undead without burning through all their spell slots to reaffirm control.

Seems reasonable to me.  Has a great ability not found elsewhere, one of the better mid level spells  and a boost for a related helpful cantrip.

If you think it is weak, you can add a few minoir benefits.

Consider making it count as a spell casting focus and/or doing +1 necrotic damage whenever you cast a spell that does necrotic damage.