Maokai shields his allies by drawing power from hostile spells and attacks, reducing non-tower damage done to allied champions in the area by 20%. Maokai can prolong the effect for 10 seconds, as long as he has mana to spend on it (Upkeep Cost: 30 mana per second). When the effect ends, Maokai unleashes the absorbed energy to deal 100 / 150 / 200 (+50% of ability power) (+2 bonus damage per damage absorbed) damage to enemies within the vortex. Maximum 100 / 150 / 200 bonus damage.
I do know Fiddlesticks can move while his ult is on and Maokai can only place Vengeful Maelstrom once, but assuming the Fiddlesticks uses the Zhonya's Hourglass active I'm sure this ult in conjunction with Runic Bulwark would nullify alot Crowstorm's damage.