Hm. I'm missing something. This seems to work AFAICT?

Well, it's not useless. But my thought is, it looks like it could attack again the turn you ninjitsu it, bolstered by the "only once each turn text", but it can't cos no haste.

Ah. I think you really want:

"Whenever [...] the first time each turn, untap Flurry Ninja. After this phase, there is an additional combat phase. Only Flurry Ninja may attack during this phase."

Ha, right, no, I see Jack's point. Yes, it does indeed look as if it can attack in the second combat. Nicely done!

Thanks.

I like jmg's suggestion in that it makes the flaw more obviously a flaw. But I think it's also suspicious to see an "extra combat step just for this" when usually it's a bit more general than that. I considered "ninjas can attack", but didn't help; would "only ~ or other ninjas" work?

I like it just affecting the Flurry Ninja, mostly because it's so frustrating. With untapping all creatures, or just ninjas, this guy still has an ability that does something relevant (albeit, he/she still belongs in this set. You certainly couldn't print it.)

But when it's only affecting this creature there's a lot more forehead slapping. At least it feels that way to me. But I'm the guy who designed Bewildebeast, so I don't know how much one should follow my opinion.

I thought it didn't work because Ninjitsu isn't a mechanic? (It's spelled Ninjutsu.)

Also just a typo, also fixed :)