Conversion converts an enemy monster as you strike it. The skill has some uses, but is severely limited. Like all crowd control skills, it doesn't work on other players, and neither does it work on bosses, Super Uniques or Champion monsters. So you can't convert Andariel and have her fight for you.
Lore[]
Through force of will and strength of steel, a noble Paladin with this skill is able to blind his enemies with the glory of the Light. After trading blows with the Paladin and facing the fire of righteousness burning in his eyes, an enemy will sometimes be struck with a divine epiphany and momentarily repent his past undertakings. So complete is the transformation, that the converted will turn to slay its former comrades.[1]
References[]
↑Diablo II Manual, page 80. Blizzard Entertainment, 2000.