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).



How It all works.
Normally the bossbar will focus on the strongest boss, if you are fighting multiple bosses and you want to display their boss bar, you can attack the boss first, when attacked, it will display their boss bar.

The destruction intensity (how quicker it takes to remove the bossbar) can be increased by their decreased defense and total health.

Trivia

 * Notice the fact that when you take a closer look at the image, it looks like an Eternia Crystal is on top-centre of it.