Predecessor Builds > Mourn > Mistmeadow's Nightmare Contagion Spam Build
Build focuses on building a decent amount of ability haste and tanky stats to survive and spam Contagion in team fights with Giant's Ring, you honestly get 100% up time with it and it's disgusting. Saphirs is there to generate more stats with World Breaker. If I'm not dropping Entropy for a situational item I get it early for the mana to help sustain Q spam once I get Giant's Ring. If its a tough match up I usually go World Breaker second. REMEMBER to charge up the Beguile ability as it gives you a flat 25% dmg reduction making you giga tank and lets the D.o.T do work. Your Combo with Giant's Ring is Q > Fully Charge Beguile (E) and in a perfect scenario while charging up you've taken hits and Giants Ring has reduced the CD on Contagion (Q), you Q again. Almost a infin rotation.
For the ulti I tend to not put a point into it early in the Offlane (put an extra point into Beguile for wave clear) unless the lane is lost, where I'll then take ulti early and look to spam ganks whenever it's up.
Build is viable in Offlane and feels the best there. It is decent in Support. Haven't played Mourn in the jungle yet, don't know if I will but you can try this build. I'd say rush Fire blossom > Megacosm > World Breaker for jungle.