Eye of Death Ideas? (Possible "The Master's Vault" module spoiler)

Hello all! I am a fairly new 5e DM continuing an adventure after finishing "The Master's Vault" module. My players are currently searching for the "Eye of Death" (an item that the necromancers want to turn everyone in the kingdom into undead) talked about in the module. I need some ideas for stats and effects that this magic item could have. Has anyone created this item or can think of some stats I could use that would be appropriate for such an item. I've thought of possibly making it sentient but am unsure if it would be appropriate for the party at this point (they'll be around level 4 when they reach the item). I'm not too good at balancing. Any help or suggestions would be welcome. Thanks!

Try possibly something that would let someone cast animate dead once per day. 

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