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

Area contains additional Legion Encounters.

Both Legions have Generals.

All Chests are replaced with War Hoards.

Legion battlefield is carpeted with purple crystals. When high ranking Legion monsters die they will shatter nearby crystals, dropping items.

Lower rank Legion monsters are upgraded to a higher rank.

High ranking Legion monsters are placed close to the Monolith.

Legion Monsters can be broken out of Stasis multiple times, increasing their Difficulty and Reward for each time they are broken out.