Boss health bar

Boss bar.
An boss health bar appears when you summon a boss, when you spawn it, you will see an red/blue (pillars-exclusive) bar with silver-like colour on each sides of it, as the boss health descends, the red bar descends, the empty part (dark red) is the depleted health. Once killed, with no transition, it will disappear, this applies to each of every boss existing in terraria (if they show on minimap), this disappears too if you click F11 (Controls).