Aberrant Manawurm

The 2/5 body is the tell. This is a defensive frame stapled to an aggressive engine, and the split is what makes the design work. Most spellslinger payoffs are fragile: they trade small bodies for big triggers and fold to a single removal spell. This one shrugs off early aggression from behind five toughness, then converts each instant or sorcery into a permanent-until-end-of-turn power spike scaled to the mana you sank into the spell. The scaling is the wrinkle worth sitting with. Because X counts total mana spent, not the spell's printed cost, a cheap cantrip nudges it while a fully kicked or overloaded haymaker turns it into a genuine finisher, and trample means the extra power lands rather than getting chump-blocked into irrelevance. That ties the creature's clock directly to how much gas your hand has left: it grows exactly as fast as you are casting, and it grows most on the turns you were already committing hard. The design lineage here is the durable green-blue tempo threat that rewards a deck for doing what it wanted to do anyway, with the toughness doing the patient work of keeping it on the board until a big spell arrives. Nothing about it demands a build-around; it asks only that you play instants and sorceries and eventually cast an expensive one.