Gah, stupid non evergreen mechanics. Ok, tribute is "Opponent can choose for this to get X +1/+1 counters on ETB)

So it's either a flying 6/6 or it brings a leviathan into play?

I like the opponent's choice part; that's a fun mechanic. "You're screwed; choose how"

And it's an ETB trigger, not a cast trigger; so you could use this to bring back another one - though I guess that only helps you with 2 more power of flying as the opponent will make the same choice.

But yowsers, this only costs 4. Feels scary cheap. But I guess you have to have found a way to discard a powerful creature to get use of it. Or, um, make an opponent discard something. This is kinda evil with Relentless Rats, isn't it?

I assume you mean Ravenous Rats.

Tribute is indeed a fascinating and awesome mechanic. This is fairly well costed; it's either a Zombify with a free, um, Dusk Imp - which is fine, but not fantastic - or a 6/6 flyer for 4, which is very scary (see how ubiquitous Desecration Demon is in tournaments) but may be less scary than the fatty in your graveyard. But like all tribute cards, when you cast it it will be whichever of the two options is less good for you. If your graveyard only contains a Disciple of Phenax? Sure, you can get that back, I've only got land in hand anyway. If your graveyard contains an Inkwell Leviathan? I'm letting you have the 6/6 flyer rather than the even-fatter even-harder-to-deal-with guy. But you need to have got that fatty into the graveyard first.

I decided on 4 for a 6/6 because it's become a bit of a "thing" in my eyes, with Desecration Demon, Abyssal Persecutor, and to some extent Alms Beast and Rakdos, Lord of Riots.