The array enveloping the first floor of Solus's tower was something they had developed in case Lith managed to find a mana geyser while being on the brink of death.
It allowed Solus to harness the energy of the mana geyser to heal all kinds of wounds almost instantly and to share part of her life force with Invigoration. The final result was a powerful healing field capable of beating death as long as the subject's mana core was intact.
In Fallmug's case, however, Solus wasn't giving him any life force. Lith refused to have her tainting her noble spirit with such a human faced monster.
Fallmug's spine recovered and so did his limbs. He was seconds away from fainting due to exhaustion when Lith used Invigoration to restore his life force. Lith wouldn't let him get any respite.