Balor

One eye, in the middle of the forehead, and the lid over it is so heavy that four men with a hooked pole are needed to lift it. This is not a weakness. It is a procedure, and it takes them a moment, and the moment is the only warning anyone gets. Whatever the eye is turned on when the lid comes up is finished - an army, a hill, a fortress gate. He is a king and he is old and he has been told how he dies, which is why he watches his own family more carefully than he watches his enemies.

The Eye: the four attendants raise the lid over one full round, in the open, where everyone can see them doing it. On the round after that it opens - 300-ft line, save Dex Quick, fail save roll 3d10+6, and a creature that fails is destroyed outright rather than merely killed, with nothing left to raise. Cooldown 4 rounds. Kill an attendant and the raising takes a round longer. Kill all four and it cannot be raised at all, which is the entire fight. Great Blow: bludgeoning, melee 15 ft, +6 to hit, 3d10+6, and a backhand, 3d8+5, as his second attack each round.

The Four Attendants: each has AC 14, 20 HP, DR 0 and no attack, and they can be targeted normally by anything that can reach them - they stand beside him in the open, holding the pole. Kill one and raising the lid takes a round longer. Kill all four and the eye cannot be raised at all for the rest of the encounter, and Balor knows it and will spend the fight keeping them behind him. Guarded Prophecy: he will not be alone, will not be flanked, and keeps his own kin where he can see them. Reduced to 0 HP he dies like anything else, and the eye goes out, and whatever it was last turned on stays gone.