­Liliana's Shade + Snake Pactmaker

Black landesearch? Didn't know that!

So, pretty clearly we have here a common creature that brings an extra card into play with it. The pactmaker also goes to neighbouring colours to find its powerup; so a is plausible here.

Alternately, we could just have a flying landbreather. Nah. Blue+Black=Green, right?

Oh, and it should have creature type 'S'

So I don't think the rules actually support memory of where mana came form; but the concept is pretty obvious for players to understand - and like the shade it has a nice "Obvious use that, oh, hang on, I could tap some funny land and make it much better" use. (Where the shade leads up from 'it boosts it more' to 'hey I can start casting nightmares')

Maybe ought to be 2/3. Also, made me think of a challenge; we've not had one recently.

...you may search your library for a land you drew mana from to cast it...

This wouldn't work, to do I think it should read something like: ...you may search your library for a land with the same name as a land you drew mana from to cast ~...