Crystalize

[serprex]

Target creature becomes a permanent

Replace active with "become a creature" costing as much as their card costs

[Chapuz]

I don't find any use for it. Got a combo or something?

You have a creature, then it becomes a permanent and loses its active ability not even all creatures can make use of it. And suddenly they become fragile to PC. In relatively fast games, Cloak may be more useful.

[serprex]

Originally suggested active cost makes this similar in annoyance to Unsummon

[Chapuz]

I also don't play Unsummon, and I don't know if anyone does.
When would you play something like this? Too soft as CC and I don't find anything good to be used in any other way.