(will comment tomorrow, g2g)
Sorry for the double post but...
I always disliked cards that break some of the "unwritten rules" (immortals are supposed to be immortal), but this anti-immortal mechanic sounds just perfect.
The only 2 problems I see to it are: maybe it's too cheap, for what it does, and I'm 90% sure I've seen this idea before (maybe it was this same post).
The cost would let you twin universe phase dragons, which was the reason why phase dragons are immortal in the first place, and also buff them if you go duo/trio/rainbow.
Apart form that, you can make this a spell if you like, it'll be like the same, and this fits more as a spell...