Similar to Thornmail, this item's passive interacts with itself. Given the lowest amount of MR possible with this item (30+40=70), then the damage taken calculation (100/170 = .588), then the raw damage conversion (400/.58), the shield will absorb around 680 raw magic damage.
The shield can block the initial burst of damage from an enemy champion and give the owner an opportunity to attack or retreat. It is very useful against AP casters with long cooldowns, such as Morgana and Ziggs.
Attack Damage champions who benefit from being at low health (e.g. Tryndamere, Olaf, Aatrox) can put it to great use. Their low-health passives combined with the item's AD effect, and the shield allows them to stay at lower health more safely.
At 30% of total health, when this item's shield activates, it provides the maximum 35 bonus AD.
Maw of Malmortius is named after a well-known community member known as "Malmortius" (also answers to "Lord Voldemortius", "Malmorpheus" and "James") for obtaining over one thousand referrals.