Sentinel

A combat-math experiment, and one that reads stranger the longer you trace the wording. The activated ability rewrites the toughness floor based on whatever this is fighting, which sounds like a defensive lifeline until you notice the details: the change is to base toughness, the effect lasts indefinitely, and the ability requires a target creature already blocking or blocked by this one. So the four-mana 1/1 body is the cost of admission to a creature that, once it has tangled with something large, holds a toughness that matches its dance partner. But the wording sets base toughness to exactly 1 plus the target's power, so it is a reset, not a ratchet: activate it against a smaller creature later and the toughness drops to fit. The free activation cost is the design tell. Legends was working out how to price a permanent's combat utility, and Sentinel hands you the ability for nothing in mana, gating it entirely behind the requirement that the body first survive into a combat relationship. The shapeshifter type line is doing flavor work the mechanics back up: a creature that takes the measure of its opponent and reshapes to meet it. The card's weakest assumption is that a 1/1 will live long enough to start activating; the modern read is a design artifact, interesting for what it shows about how toughness-modification was conceived before static anthems and combat tricks took over the job.