The Fecund Seal is a Unique Ring in Diablo IV Season 15. It comes from Harbinger of Hatred (Lair Boss loot table); Harbinger (Lair Boss loot table, boosted class-unique drop).
Unique power
Sigil of Summons deals [100 - 140]%[x] increased damage and you can summon up to 2 additional Lesser Demons with 10 kills outside the ritual.
Affixes
Willpower ([9-11]%)
Skill Ranks to Sigil of Summons ([2-3])
Where it drops
Harbinger of Hatred (Lair Boss loot table)
Harbinger (Lair Boss loot table, boosted class-unique drop)