Nyxborn Unicorn

Two mechanics that share a spine sit together here for the first time in a clean package: mentor wants to attack, bestow wants to attach, and both are ways of pushing power onto a creature you already control. The unicorn hands you the choice each time it lands. Cast it as a 2/2 and it comes swinging with its own mentor trigger, feeding a counter onto a smaller attacker. Pay the bestow cost instead and it stops being a body: it wraps around a creature for +2/+2 and lends that creature mentor, so the host grows the rest of your board every time it attacks. The elegant part is that the two modes reinforce the same plan rather than competing. Mentor is a fragile ability on a two-power creature, because it needs a lesser-power attacker to target and can be picked off before it does much work; bestow answers that fragility by relocating the whole package onto something bigger, harder to kill, and already committed to combat. And because a bestowed creature becomes a creature again when its host dies, the mentor engine survives removal that would otherwise strand a plain Aura in the graveyard. It is a small design, but a tidy one: an enchantment creature built so its two keywords cover for each other's weaknesses instead of asking you to pick a lane.