If we were to pick the strongest among the many weapons Diarmuid Ua Duibhne wields, it could only be this fearsome demonic sword. A blow bringing certain death, another certain victory. Unsheathing the demonic sword, Diarmuid is gifted with certain victory, bringing defeat and death to the opponents. The demonic sword Moralltach, which can almost control even destiny, was awarded to him by the Celtic God of of the Sea and Underworld Manannán. Manannán is for sure the deity who gave the Noble Phantasm Fragarach to the Sun God Lugh, he possessed many Noble Phantasms and was a being who donates.
When he releases its True Name, he makes use of a super-jump beyond human comprehension to perform a skydrop attack. At this time, Moralltach, resembling the "leg of the god Manannáns" from the legends, becomes the three blades to tear the enemy into pieces.