Although the Rat King had been successfully incapacitated, mauled, crippled, and brought to the brink of death, Toz didn't relax. Despite having every piece of its body pierced by the metal spikes, the Rat King still wasn't dead. Not even having its eyes and brain punctured instantly killed it.
But even if it refused to die, it at least couldn't move, and it didn't seem to be able to use magic either as it just twitched and wriggled in the pitfall. However, even with its entire body torn open by the spikes, the Rat King didn't bleed as much as one could expect. Thanks to the Rat King's natural regeneration, its wounds were trying to close up around the spikes, keeping the blood inside.
The Rat King definitely couldn't move or think or get out of the current sticky situation it found itself in, but if it continued healing, there were no guarantees it wouldn't be able to absorb the mana in the spikes as soon as it began recovering.