Sukuna
Sukuna is a Dragon Lord of the sea who's been nursing a grudge for over five centuries. Tomoe ripped out his right eye and destroyed the north gate of Ryuuguu Castle, and Sukuna has been dead set on revenge ever since—literally keeping count down to the second for 526 years, 28 days, 2 hours, 18 minutes, and 4 seconds.
His eyes are legendary among those in the know. They're called the Longevity Elixir, and drinking them supposedly grants immense power—which explains why they're such a big deal and why Sukuna's missing one probably doesn't help his mood.
The turning point came when Sukuna rescued a drowning girl from the sea. That act of mercy is what finally led him to cross paths with Tomoe again, setting the stage for the confrontation he'd been waiting half a millennium to have.
Content compiled by AnimeList.moe from publicly available sources.
