The Suspicious Looking Tentacle is a summonable light source that is dropped by the Moon Lord in Expert Mode. When used, it spawns a Suspicious Looking Eye to produce light, detect enemies, and highlight ores and treasure. Its eye expands for treasure and contracts for enemies, in addition to pointing in the direction of the target. It resembles a True Eye Of Cthulhu, one of the attack phases in the Moon Lord boss battle.
Despite what the name suggests, this doesn't summon a boss.