Katakuriboh [Written]

[Thomas★Zero]

Katakuriboh 
lv 1 DARK
Fiend/Effect
You can Special Summon this card (from your hand), by tributing 1 monster your opponent controls (this is a Quick Effect). You can only Special Summon "Katakuriboh" once per turn this way. This cards ATK and DEF become equal to the tributed monsters ATK.
ATK/? DEF/?

[Darj]

Got to say no to this one. It may not be Nibiru but it is much more flexible and it doesn't even give the opponent anything in return. 

I get what you were going for with the first effect: you want to make a "quick inherent Summon", that is, without starting a chain, similar to Kaijus. Sadly that isn't a thing in YGO, as there is no precedence for monsters that Summon themselves during the opponent's turn as inherent Summon, and while I support that card texts can override mechanics, I feel it's cheap to do so for the sake of the extra power. Not to mention the conflicts with rulings and timings that it would ensue. For starters, built-in/inherent Summons by mechanics are unclassified effects, don't start a chain, and are only useable in open game states, so treating it as a Quick Effect makes little sense without a PSCT that overrules the aforementioned traits of built-in Summons. So IMO just keep it simple and stick with making it a from-hand effect that starts a chain, like Nibiru or Phantazmay.

Even then, it would remain too good, IMO. I would bring fix suggestions but atm can't think of anything that doesn't conflict with Nibiru: if it's easier to set up, then it outclasses Nibiru; if it's harder to set up, then you may as well play Nibiru instead. You can try exploring different conditions instead (e.g. when an opponent's monsters battles, during a turn you took damage, to throw some ideas associated with the Kuriboh archetype), in an attempt to cover what Nibiru doesn't, and thus having a different niche or application. But I do suggest to nerf the Token effect. Depending on which condition you add, the Token effect may still need a nerf, since removing a threat or stopping a combo and getting a monster that can potentially be a beater in the process sounds a bit too overpowering to me.