Cost: Deplete this card. Pick a character with the dragon or seraphim subtypes.
Effect: The picked character gains 1 life.
If a dragon character you control is destroyed, you may destroy this card instead and put the dragon character back into play, depleted (treat it as if it has received no damage this turn).
Flavor: "I kill when I wish! I am strong, strong, STRONG!"
Adding yet more theme support. Dragons are cool and need some love. ^_^ I know it's breaking flavor a bit to add only life to a Warlord character, but I think that's most logical for a cave and I don't want to give Plunging Shriever and Deadly Striker more strength.
So without any dragons this card is totally worthless. Seems like a fun card for a draft. I'd probably do it something like this.
Cost: Deplete this card. Pick a character. Pay 2.
Effect: The picked character gains 2 strength. If the picked character is a dragon character it also gains 1 life.
If a dragon character you control would be destroyed, you may destroy this card instead and deplete the character. Treat it as though it recieved no damage this turn.
I knew you were gonna lay that draft thing on me... ^_^;
Well, I like your idea except for the attack giving part. I think it's more logical to give any character life as a cave is primarily a defensive structure. Perhaps eliminate or decrease the cost then too, at least for the activation.
The dragons obviously gives strength to the other dudes, and use their cave to boost their defenses. I don't think the strength increase is too good. It's basically the same as 614nd P3pp3r with one less life, and on a single character. It's reusable, but much more expensive, and not that hard to shut down.