When to buy 
Mercurial Magnum Mercurial Magnum 6,400 souls Tier 4TechPassive +15s Cooldown +7 Spirit Power +22% Fire Rate +12s Buff Duration +100% Bullets Reloaded +14s Charge-Up Time +20% Max Ammo Builds from Quicksilver Reload
Buy on hybrid heroes with heavy spirit investment who shoot after casting.