Deadly hits are like critical hits, but unlike critical hits which deal 200% damage, Deadly hits deal 300% Damage
How to get Deadly hits
Non damaging Deadly hits
It is possible to get a deadly hit when using the Light healing spell,target will be healed at 300% of the base healing number; so if a +45 Heal got a deadly hit, it would heal +135 instead. It is also possible with Prayer Of Protection spell, you will get +75 off Prayer Of Protection 2 and possibly, a stronger shield.
I've yet to see a critical shield, so im thinking that isn't possible.
Also it is unconfirmed if this works with any other spells.