Eye of Cthulhu Relic

Relics only drop in Master Mode. Kill the same boss on Classic or Expert as many times as you like and nothing appears, so a shelf of relics is a straight record of which fights you cleared on the hardest setting. Event relics have an extra condition: the Dark Mage and the Ogre hand theirs over only in tier three of the Old One's Army, so clearing the earlier tiers will not produce Dark Mage Relic or Ogre Relic no matter how many runs you do.

Nothing mechanical. Like a trophy, it changes no drop, no difficulty and no boss behavior. It is a gold figure of the boss hovering over a pedestal with a slow particle drift, and the full set covers every boss plus the event minibosses, which is a long term collection project across a world or several.

The glow is reflective rather than emitted. A relic in a dark room just sits there, and it lights up only when something else is throwing light at it, so a torch or lantern placed nearby at night is what makes the effect visible. Paint is the other trap. These behave like stone and mud, meaning ordinary paint barely registers and only the deep variety shows a real color, and even then only the plinth takes it. The gold figure on top stays gold whatever you do, so you can match the base to a room and no further. If you are lining several up in a row, note that the Skeletron and Lunatic Cultist pieces along with Skeletron Prime and the Ogre look slightly off center against their bases, so a perfectly spaced shelf will still read as a little crooked.