Most Malphites like going with, Domination: Dark Harvest, Sudden Impact, Eyeball Collection, Ultimate Hunter
They all proc with his ultimate.
Dark Harvest: Extra Adaptive AP dmg when enemy below 50% health AND 'harvest souls' gaining more damage to Dark Harvest.
Sudden Impact grants 7 Lethality and 6 Magic Penetration, mix that with his W and E (AoE) right after.
Eyeball Collection: Each Take down drops eye ball to collect that gives 2 AP per eyeball. Also every 10 eyeballs collected grants 10 adaptive AP
Ultimate Hunter lessens Cooldown per take down (20% max CDR)
So yeah hes a monster to be reckon with, but you have to stop him from farming. Also pre 6 is prime time to take him down as much as you can. Once he hits 6...even IF he is dead weight...he can still ult in, die, and his team takes out those airborne.
PS. Him and Yas are a nightmare once they both hit 6 as Yas doesn't need to land his tornado if Malphite ults in.
P.S Sub rune most Malphites like 'Coup de Grace' in Precision to do extra damage to enemies below 40% health (W+R+E+AA~~ Q Survivors to stop their retreat).
Its the runes that really make him so destructive.