The wording is kinda odd on this one. I'd reccomend:

"You may cast commanders without paying their mana cost.

Commanders you control have no abilities and can't attack or block."

But I'm not sure what this card is supposed to do? You get your commander out, but it leaves when this leaves? Maybe make some changes here. It doesn't really do much as is.

The Core turns into Darcy with a suitable host, and there's good news and bad news for it. the good: it's technically always available in their opponents command zone. bad news: It can't get the creature unless it's it's on the battlefield. The Rejuvenation Tank steals an opponents commander and locks it away for later use, or you can use it to heal your own commander. I just realized that you can't target a commander that's in the command zone, so I'll change it up real quick

Ooh, I didn't notice that it can snag others ccommanders.thats actually pretty neat