Holy Seals are one of the rarest units in the game, being a rare encounter in a rare biome, the Heaven mini-biome that only spawns in floating island Portals. Due to their extreme rarity it's arguably worth bringing a MindBender just to ensure a successful capture.
In any case, they're not recommended for getting access to SealHaven; if anything, they're easier to find there than in their native habitat.
They are an Elite with high magic growth and a special passive that gives all seals in the party +1 AP. They have no fixed skills but have a huge pool of potential skills, mostly magic skills based around light and healing.
They can be sold for 1000 gold at Medium size and for 3000 gold at Enormous size. Every level above 1 additionally increases their price by 5%.
- It's unconfirmed but rain may increase the odds of finding one.