Dammon tells Karlach that the infernal engine that makes up her heart is just too hot to function in the Material Plane despite his best efforts to fix it. Unless Karlach returns to Avernus, which is something she swore to herself to never do due to Zariel still hunting her, it's only a matter of time before the engine overheats to the point of it burning her from the inside out.
Example of:
Your Days Are Numbered
Dammon tells Karlach that the infernal engine that makes up her heart is just too hot to function in the Material Plane despite his best efforts to fix it. Unless Karlach returns to Avernus, which is something she swore to herself to never do due to Zariel still hunting her, it's only a matter of time before the engine overheats to the point of it burning her from the inside out.
