I was watching Amumu Top the other night and we were discussing builds. Some of those in the lobby did not realize the synergy between the two items.

The frozen heart aura counts as a movement-impaired debuff for the purpose of activating liandrys double damage tick (4% of the target's current health per second). If you dont believe me go into practice tool and test it for yourself. I did just to make sure it wasnt fixed at some point.

I believe this is why FH is core on amu as your main armor item. If enemy team is made of more magic damage then build the Abyssal Mask first (another item i believe to be core to amu as it is tank 10% more magic damage).

Example build (in no particular order) :

Cinderhulk (Deals 12 - 29 (based on level) magic damage per second to nearby enemies)

Frozen Heart (aura activates liandries 4% current health per second)

Abyssal Mask (Nearby enemy champions take 10% more magic damage (500 range))

Liandry's Torment (Dealing spell damage applies a damage-over-time effect that deals bonus magic damage equal to 2% of the target's current health per second for 3 seconds. This bonus damage is doubled to 4% of the target's current health against movement-impaired units)

amumus new passive (Cursed target receive 10% bonus true damage from all incoming magic damage, calculated before magic resist reduction)

The other 2 items in your build can be whatever boots you feel the need for and a situational item.

very tanky build with some very deceptive damage.