Nekron
As the embodiment of Death, Nekron first appeared in Tales Of The Green Lantern Corps (Vol 1) #2 in 1981. He is Ruler of the Dead Zone – which borders on Hell, Limbo and Purgatory – and draws his powers from the souls of every being that ever died. He is arguably the most powerful of dark forces in the DC universe, and is able to warp reality, resurrect the deceased, shoot lightning bolts, increase his size and kill instantaneously, with a single touch.
Having no soul, Nekron is invulnerable to divine wrath and spiritual judgement, and has engaged in battle with everyone from Wonder Woman to Superman, to Green Arrow, as well as the Green Latern Corps. It is the Corps that first defeats him, when he is seeking to gain access to a rift between dimensions, which would allow him to pass to the Realm of the Living using a recreated being as a proxy.
The proxy – Krona – comes into conflict with the Green Lantern Corps, and Hal Jordan specifically, who travels into the Dead Zone to encourage deceased Green Lanterns to rise up against Nekron.
In the New 52 reboot of 2011, Hal Jordan harnesses the villain Nekron to defeat the corrupted First Lantern Volthoom. Though he had previously been trapped in the Dead Zone, Jordan’s negotiations, and sudden working relationship with Nekron lead to his release back into the living world. Once Volthoom is beaten, Jordan is returned to life himself and banishes Nekron back to the Dead Zone, to plot another scheme.