You can trade in your collected Seals at any Seal Trader in exchange for special items. Seal Traders can be found at the Protector's Enclave market, or at certain camps outside the Enclave.
Seal of the Unicorn was earned in elite dungeons. Was also obtained from the Bounty Masters in Sharandar for 40 or Dread Ring for 40.Also used to be available by completing a Tier 6 adventure in Sword Coast Adventures, yet no longer available.
These used to be exchanged in Protector's Enclave to Phileas Gram the Unicorn and Drake Seal Trader for Tier 1 Epic dungeon equipment. They were mostly removed from the game with the release of Elemental Evil Exchanging for Rough Astral Diamonds near the end of it's life until module 9 when the [Seal of the Adventurer] was added.
One of many currencies once minted in Neverwinter to award heroic deeds. Thanks to merchant's guild lobbying, this coin and others like it have been replaced with a single coin—to the chagrin of cambists and the relief of adventurer and merchant alike.