So, I'm making this stone mask for a campaign, based on the one In Jojo that Dio uses to make his Zombie Vampire minions/agents. If you're unfamiliar with how the stone mask works, or haven't seen JJBA, basically it's this mask that when put on others can turn them into vampires. I was thinking of allowing the player who got the item to use it on enemies, and turn them into minions, which they can summon at will. The maximum was 3 at a time because I thought maybe that would make it less overpowered.

It's going to be loot for a boss I'm working on, but I hate 2 main concerns. 

1. I feel like the stone mask is a pretty powerful item, and I don't want it to be too unbalanced. I don't want it to be a case where there's one player in the party who's just way stronger than the others.

2. I'm not particularly sure how to come up with balanced stats for the item. I just don't know where to start. I have the description down, and I understand how it's going to work but I just don't know how I'd go about determining things like modifiers and conditions

Is it even possible to carry this out or should I abandon the idea altogether

Summon minions at will is extremely powerful, I'd suggest abandoning it for now.

Sounds a lot like animate dead to me. I'd take guidance from that spell or just straight up have the item grant them X uses of AD per day.

At any rate, the minions should just be one fixed stat block and relatively weak.