And for what purpose may it be used?

The Erolin Device is nowdays the swarm of heat orbs on the left of the interface. It used to be a good looking device one day, in the top right corner of the interface, but now it got a more permissive shape. 

The heat orbs can store the heat you generate when walking or fighting or doing other things. This heat can be reactivated by clicking a glowing orb, and it becomes again active heat that drops one point per second and can be used to cast spells (for example).

 

You can get more heat slots in the shop or on christmas tree, or other rewards. 

One heat slot can store only a fixed amount of heat, not more not less. If you don't have enough heat to load it, it will not load.

To store your heat in the Erolin Device, click the small "store" bubble on the left. it will make a mechanical sound