Why at the end of your combat? It seems like an odd time to do it.

Either end of your turn, or end of opponent's combat would seem more natural.

All of the current 2 color pairing cards are designed to work well with the treasonous mechanic. Since the most common source of a treasonous proc would be from combat damage I chose the end of your combat.

I could see the argument as to why it should be less restrictive. I guess originally I feared that the effects may be too powerful if it was open to proc any time. I would love to hear your input on this?

It's stealing 1 life a turn, at most. I can't really see a way that could be too powerful.

Power wise this compares extremely poorly to Drana's Emissary. Same exact mana cost, -1 power, no flying, and drain made extremely conditional. At least it has a chance of triggering on the turn it drops.

The mismatch of typing and the art makes me confused. This to me, looks like an Imp, which I would assume to have flying due to its wings. "Beast" is a strange choice all in all.

How about every combat rather than just yours?

Just increase the power and toughness, too.

The most problematic issue with this in the "power" department is that this gains a lot more life in multiplayer, but referencing Drana's Emissary also gives you a better wording for that.