Halo sends out an expanding ring of Holy energy that damages enemies and heals allies it passes through.

As Archon, Halo becomes a major raid cooldown: it pulses for a long buff window, generates Surge of Light procs, and stacks Resonant Energy that increases the priest's healing output. Outside of Archon it functions as a smaller short-cooldown effect.

Generally not worth assigning, Halo is mostly just fired on cooldown, and outside of the Archon hero tree there is no reason to plan a slot for it at all.

With Archon, if you do assign it, line it up just before Apotheosis or Divine Hymn for the synergy. Even then, don't hold it longer than ~30 seconds, as letting it drift causes a noticeable loss in healing throughput.