Here are some Lair Actions I came up with... thoughts?
Lair Actions
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Soul Tether. Spirits from the underworld move throughout the lair.
All enemies within the Lair must make a DC 20 WIS saving throw. On a failed save, a thread of the creature’s soul is tethered to The Master. Whenever The Master takes damage from any source, the tethered the damage is divided equally between all souls tethered to The Master. The damage dealt to these souls is considered force damage.
If a creature succeeds on this saving throw, it is immune to this effect for 24 hours.
You do NOT want to be the only PC to fail the saving throw! This ability gets more dangerous the more that save (assuming at least one fails).