Predecessor Builds > Zarus > Zarus Jungle
Basicly Malady is core. I like Perforator 2nd if I'm ahead. After that, it's just reading the enemy team, who is fed, what do I need to have late game for potential threats. If I need anti heal I figure out if I need tainted bastion or tainted guard, and maybe I just need serrated blade because my dmg can apply the anti heal best. If they have a lot of attackspeed i get frostguard. If i need sustain i get mutilator or some healing buff items, sometimes even Nightfall when im feeling funky. Nightfall + Draconum is deadly and super fun.
2 full damage items is usually enough damage to take out enemy carries.
Start stacking your malady and your ult, make sure you weave in and out of combat, looking for picks in teamfights. As that is where he is strongest.