Interesting, but specify whether the defence returns after each turn. Blocking 25 damage each turn could be seen as offensive to Armagio-lovers, so just might want to specify.
also, this would be used in CC decks, so probably lower the defence a bit.
Increase the upped's cost by 2, and maybe throw in a "divided by 2" thing just to make sure it's not too much. I would love to insta-kill titan and become untouchable. With Siphon Life, it's not just a dream.