Requires 50 points in Retribution
An instant weapon attack that causes Holy damage to up to 4 enemies within 8 yards. The Divine Storm heals up to 3 party or raid members totaling 20% of the damage caused.
First of all, understand that Paladins are a glorified auto-attack class. Unlike other melee classes with a plethora of strikes (sorry, Enhancement Shamans, I know you're in the same boat), Paladins rely on the swing timer and insert the occasional, short-ranged Judgement in between. Retribution Paladins have one additional button to press, the 41-point Crusader Strike, but otherwise it's an auto-attack affair. This new spell, the 51-point granddaddy in the Retribution tree, hits numerous birds with one stone -- it's another activated strike, raising DPS; it can hit multiple targets; and it provides minor raid utility with AoE healing.
More than anything, Divine Storm results in a significant DPS increase. It procs Seal effects, resulting in huge potential gains in mana or health through Seal of Wisdom or Seal of Light when used against multiple mobs and a phenomenal jump in damage when using damage Seals. This is a new design as Blizzard revealed that "(Seal) effects can be triggered from all weapon based special abilities." It can also crit on each hit. Wrap your mind around that for a while. Need to change your pants yet?
The only downside is that it costs 20% of base mana, which can quickly deplete a Retadin's paltry mana pool when used every time its up without a proper mana regeneration mechanic (although Judgements of the Wise has been reportedly hotfixed...). It's also notable that it can be cast even without a target as it is considered an AoE spell.
Divine Storm is Whirlwind Plus. The healing is a really just a bonus, scaling with the number of opponents and inversely with the number of party or raid members. For best results, use on multiple mobs while solo. The spell is a godsend for Retribution Paladins, providing a massive -- if not necessarily sustainable -- DPS boost. Coupled with the new and improved dual-scaling Consecration, it delivers an AoE viability Retribution always lacked.
Unlike Crusader Strike, Divine Storm also deals pure Holy damage, ignoring armor mitigation. Against a single target, it's generally inferior but is otherwise the better spell in all other respects. It can also be directly improved by the talent Righteous Vengeance. While Crusader Strike remains the cornerstone of the Retribution DPS cycle, Divine Storm is a solid addition to a Paladin's ret-pertoire. I know, I make bad puns. It's really hard to concentrate while squealing with schoolgirlish glee.