I saw someone use him in a game, and he was changing every team fight in our favor. Basically Malphite's E + Randuins + Frozen Heart Aura, really hard to face especially when you can ult right in the middle of a fight and immediately get all your debuffs on. Tried it once, seems really legit. I tried to add a wardens mail but game ended too quickly. Can this work as much as I think it can?

Against AD heavy teams, yes.

Its a great counter to Wukong and several other attackers, and a supergood counter to Jax.

Malphite is a really strong pick in general.

He has amazing initiation, becomes really tanky, completely shuts down AS bruisers, has decent pushing power, and can slow people down for your team to catch up to and kill.

Also, hitting a Q on a powerball Rammus is freaking awesome.

If you have Randuins you already have Warden's Mail.

Overall, not a bad idea, especially considering there are plenty of characters whose biggest counters are AS slows