A large pile of inanimate material magically brought to life. Lava golems are made of low temperature magma and they use the magic powers contained in them to summon fireballs and similar projectiles to throw at their enemies.
Power and resistance of any particular golem depends on its size and the amount of contained magic force.
As a golem wears out, if it is very big, it may spawn several smaller ones in the process which continue their lives independent of their originator.