Morlun, Devourer of Spiders
The X here is doing double duty, and that is the whole tension. Pay X and you get two independent payoffs scaled off the same number: a lifelinking body sized at X+2/X+1, and a burn spell that hits an opponent for X as he enters. Most X-cost creatures ask you to choose between a threat and a payoff; this one refuses the choice, buying both with one mana investment. The counters arrive as a replacement effect ("enters with"), not a triggered ability, so there is no priority window in which removal can catch a bare 2/1 before it grows: whatever you paid, the body is already that size the moment it hits the battlefield. The damage, by contrast, is a triggered ability, and it points at a player rather than a permanent, slipping past blockers and creature-based defenses to push straight on the life total. But it fires once, on entry, with no honest way to reload it. Recast and you pay X again; blink Morlun and he returns with X=0, dealing nothing. The whole design lives in the arithmetic. At low X he is a cheap, disposable threat with a nudge of reach; at high X he is a haymaker that drops a life total by double digits while leaving behind a body large enough to survive the swing and start attacking. Lifelink knits the two edges together, since both the entry burn and the attacking body funnel their damage back into your own life total.
