http://www.youtube.com/watch?v=ZXRb6nPcx10

This card is broken in half, is what I'm trying to say.

tap ability instead?

Any land with a basic land type is scary as heck, because lots of stuff can be used to search for it.

Any land that is strictly better than a basic land, is too good.

This is both. Now let's read the abiliy. Which is... confusing? Is it meant to be two abilities? Anyway this lets you, um, kill a utility creature and pump a blocker every turn? That's a kinda good ability. If it was on a land with no other abilities.. if it had some significant downside maybe? A land that's also usable for mana? Heck no.

This is Legendary, so it's not strictly better. Wizards have done this themselves - see the Eiganjo Castle cycle.

Mind you, this certainly is exceedingly good. A land that turns any of your creatures into a... Grandmother Sengir, it turns out... is very strong indeed, if only because lands are so hard to remove.

I think if Wizards were printing this, they certainly wouldn't give it the type Swamp. I actually wish they'd use basic land types a bit more on nonbasic lands, but this might be a step too far.

Just how much extra value you can get on a land is difficult to guage. Mostly, because there's only one cost ever for lands, and Wizards, and designers have often tried to stretch that cost to fit their designs. That said, for non-etb tapped lands, I generally look to Cathedral of War as a guideline. It pretty much tells me that, if I'm okay tapping the land for colorless, that a good land can end up with the same ability that a good 1/1 creature can get (Akrasan Squire).

That being said, assuming this land tapped for colorless for a second, we'd have to be cool with this creature for that ability to be useable:

­
Creature - Human Shaman
­, : Tap target untapped creature. It gets +2/+1 until end of turn. Target creature gets -1/-1 until end of turn.
1/1

That's a strong ability... but I don't know if it's over the top. Kind of hard to figure out, really. I'd guess that Prodigal Pyromancer would cost if it activated for as well. Requiring an extra creature... maybe. Maybe that's balanced. But I got to agree with Vitenka's original assessment. Balanced or not, it's a mess. It certainly doesn't scream 'Mythic' to me. There are way too many knobs.

If the activation cost a little more, (about ) and the card gave a straight up bonus of -1/-1 and +1/+1 with no hoops, then I could see this as a land that adds colorless. That would probably be a fair 1-cost creature. I still don't think that land would be Mythic, though. It lacks shininess. It'd make a fine rare, however.

Added ETB tapped.