At least one Modifier will always apply when spending Threads. Spending more Threads improves the chances of multiple Modifiers applying.
Abyss Modifiers which can be applied by Threads:

An Abyssal Pit in the area will spawn an Abyssal Consort.

Area will contain additional Abyssal Pits. This Modifier can occur up to three times.

Rare Monsters spawned by Abyssal Pits will spawn in pairs.

Abyssal Pits will fire Mortars which spawn Bone Hoarders on impact.

Rare Monsters spawned by Abyssal Pits will have an additional Abyssal Modifier.