Hawking radiation. Look it up, black holes slowly lose mass due to quantum effects near to event horizon.
Quantum instability can cause particle-antiparticle pair production "out of thin air", even if there's not enough energy available, but those usually annihilate each other almost instantly so energy is still conserved on long term. However, if you put event horizon next to where the pair sprung up, one of them can fall into black hole and the other can escape. In that case, they cannot annihilate each other, and in order for the energy to be conserved, the black hole needs to lose that energy. Effectively, the black hole emitted a random particle.
If a black hole has no matter to feed on, even though this process is extremely slow for stellar mass black holes, it means they will eventually evaporate away through Hawking radiation.