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

Rituals in area will be Nameless.

Rituals in area will spawn a friendly wisp. Standing near the wisp will cause you to gain increased Tribute from slain Enemies.

The final Ritual will transform into a Bodach's Ritual on completion, which can then be run again for further Tribute.

Completing a Ritual encounter in area will grant all players a Blood Shrine buff.

The final Ritual encounter in area will allow you to sacrifice your Tribute to call forth a champion of the King.