Brightmare

Lifegain on entry is the most passive thing white does, a number that ticks up while the board sits exactly where it was. The tap clause pulls this Unicorn out of that mode. The same trigger that pays you life also taps a creature down, and because the life gained scales to that creature's power, both effects point in the same direction: the fattest body across the table is simultaneously the biggest life swing and the creature you most want out of the way. That inverts the usual instinct to gain life off your own board, since the payoff comes from aiming the trigger at the opponent. The tap is a one-shot, not a lasting lockdown, and this Unicorn arrives at sorcery speed, so the timing lives on your turn: tap down a blocker before you swing, buying a clean attack for a single turn cycle before that creature untaps on its controller's next step. The "up to one" wording is doing quiet work. A mandatory target finds one wherever a legal target exists, and this creature is always a legal target on the battlefield, so dropping the optional clause would not fizzle against an empty board. It would force you to tap your own creature and gain a trivial amount of life. The "up to one" is what lets you simply decline. A modest body handling two jobs off a single entry, the kind of undemanding defensive piece that rounds out a white curve without asking the deck to bend around it.