Spawns a random Harbingerfamiliar that changes every 10 seconds. All harbingers deal contact damage to enemies and periodically spawn locusts if Isaac currently doesn't have any, though each harbinger behaves slightly differently:
Famine wanders around the room, leaves a trail of slowing black creep, deals 0.5 contact damage per tick or 15 damage per second, and spawns yellow locusts every 3 seconds that deal double Isaac's damage and slows enemies.
Pestilence wanders around the room, leaves a trail of green creep that deals 1 damage per tick or 30 damage per second, deals 0.5 contact damage per tick or 15 damage per second, and spawns green locusts every 3 seconds that deal double Isaac's damage and poisons enemies, which deals double Isaac's damage over time.
War chases enemies, moves faster than the other Harbingers, deals 1.5 contact damage per tick or 45 damage per second, and spawns red locusts every 5 seconds which detonates upon impact with enemies, dealing double tear damage for the impact of the locust and 60 damage for the explosion. The explosion synergizes with multiple bomb related items.
Death chases enemies, deals 1.5 contact damage per tick or 45 damage per second, and spawns black locusts every 3 seconds that deal quadruple Isaac's damage, applied as two hits of double damage.
Conquest wanders around the room, deals 0.5 contact damage per tick or 15 damage per second, and spawns two white locusts at a time every 2 seconds that deal double Isaac's damage.
Conquest ignores the locust spawn timer.
Notes
If Conquest is on top of an enemy when Isaac has no white locusts, it will spawn locusts at an incredibly high rate and deal a lot of damage.
If an enemy is large enough to stand on multiple segments of Pestilence's creep trail, the damage from each pool of creep will stack.
The locusts that War spawns can damage Isaac if they explode near him.
This makes it possible to gain multiple varying horsemen and gain all of their locusts at one time
Synergies
BFFS!: Increases Horsemens' size and doubles their contact damage. Does not affect locusts spawned.
Bomber Boy: War's locusts explode in a small horizontal cross-shape pattern.
Box of Friends: Creates another random horseman that can be different from the original.
Brimstone Bombs: War's locusts will shoot out four brimstone beams in a + formation after hitting an enemy.
Glitter Bombs: Locust explosions have a chance to spawn pickups or trinkets.
Hive Mind: Increases locusts' size and doubles their damage.
Red locusts’ impact damage is doubled and the explosion damage is increased to 85.
Sad Bombs: War's locusts fire a ring of ten tears outward upon detonation, which are affected by Isaac's tear modifiers, and deal damage equal to 15 + 0.6x, where x is Isaac's Damage.
Interactions
Fish Tail: Despite working with the locust trinkets, it does not double locusts from any of the 7 Seals Horsemen.
In the original mod, Conquest was able to horizontally charge with multiple copies of himself, just like the real boss.
The 7 Seals is a phrase in the Book of Revelation that refers to the 7 symbolic seals, which explains why it summons miniature Harbingers.
If all 5 Harbingers were defeated on a save file before Booster Pack #5 was installed, defeating any normal boss will unlock 7 Seals.
7 Seals was given a new sprite in Booster Pack #4. The harbinger familiars also got new sprites, based on this Steam Workshop item by Quartz.
7 Seals is one of only five items to appear in both the Angel and Devil item pools, the others being Duality, Lil Delirium, Void and Purgatory.
Although the Headless Horseman is a horseman, it isn't represented in the 7 Seals roster.
Its achievement popup graphic was changed in v1.7.6; the previous one resembled its Steam Workshop sprite, and the new one resembles its actual in-game appearance after it officially released.
The mini harbinger familiars use the pre-repentance designs of their boss counterparts.
Each use of Glowing Hourglass that does not move Isaac to a different room from the one he is currently in (such as by teleporting to the previous room then using it again) will duplicate the locusts.
Famine wanders around the room, leaves a trail of slowing black creep, deals 0.5 contact damage per tick or 15 damage per second, and spawns yellow locusts every 3 seconds that deal double Isaac's damage and slows enemies.
Pestilence wanders around the room, leaves a trail of green creep that deals 1 damage per tick or 30 damage per second, deals 0.5 contact damage per tick or 15 damage per second, and spawns green locusts every 3 seconds that deal double Isaac's damage and poisons enemies, which deals double Isaac's damage over time.
War chases enemies, moves faster than the other Harbingers, deals 1.5 contact damage per tick or 45 damage per second, and spawns red locusts every 5 seconds which detonates upon impact with enemies, dealing double tear damage for the impact of the locust and 60 damage for the explosion. The explosion synergizes with multiple bomb related items.
Death chases enemies, deals 1.5 contact damage per tick or 45 damage per second, and spawns black locusts every 3 seconds that deal quadruple Isaac's damage, applied as two hits of double damage.
Conquest wanders around the room, deals 0.5 contact damage per tick or 15 damage per second, and spawns two white locusts at a time every 2 seconds that deal double Isaac's damage.
Conquest ignores the locust spawn timer.
BFFS!: Increases Horsemens' size and doubles their contact damage. Does not affect locusts spawned.
Bomber Boy: War's locusts explode in a small horizontal cross-shape pattern.
Box of Friends: Creates another random horseman that can be different from the original.
Brimstone Bombs: War's locusts will shoot out four brimstone beams in a + formation after hitting an enemy.
Glitter Bombs: Locust explosions have a chance to spawn pickups or trinkets.
Hive Mind: Increases locusts' size and doubles their damage.
Sad Bombs: War's locusts fire a ring of ten tears outward upon detonation, which are affected by Isaac's tear modifiers, and deal damage equal to 15 + 0.6x, where x is Isaac's Damage.
Fish Tail: Despite working with the locust trinkets, it does not double locusts from any of the 7 Seals Horsemen.
Duality, 
Lil Delirium, 
Void and 
Purgatory.
Bug!
Glowing Hourglass that does not move Isaac to a different room from the one he is currently in (such as by teleporting to the previous room then using it again) will duplicate the locusts.
Locusts 
The Binding of Isaac: Rebirth
