In Three Houses, the Seiros Shield is a shield left behind by Rhea in the Holy Tomb for Byleth in nearly all routes except the Crimson Flower route, where it remains on Rhea's person.
In Three Hopes, the Seiros Shield in standard play files is unobtainable, as it is worn by Rhea, and can only be obtained by recruiting Rhea via the Renown Shop in Shez's Journal upon beating any given route, and raises Defense by 5. When the Seiros Shield's power is unleashed at the Blacksmith, its Def boon is now 10, and the attributes Divine Glow and Monster Aegis is unlocked. Divine Glow greatly restores the wielder's HP at regular intervals, while Monster Aegis halves all damage taken by monsters.
Despite not appearing as an equipable item in Heroes, the Shield would make an appearance in Seiros' artwork for said game.
In Three Houses, there is no available unit that has an innate Crest of Seiros that can take advantage of the Seiros Shield's healing effects: Rhea is unplayable, Jeralt is unplayable and long dead by the time one gets to the War Phase, and Edelgard is playable in the one route where the Shield is barred from the player.