He is secretly a night elf. Disguised as a murloc, he was sent to observe the Winterfin murlocs. Clothed in a poorly fashioned murloc costume, he hoped the murlocs would not notice him. However, a disaster caused him to aid the Winterfin tribe and they adopted him as their "king".
He offers players several quests pertaining to the murlocs. Every murloc that considers Mrgl-Mrgl a king is presumably friendly to players, and can repair armor, sell food and drinks, and even give out quests themselves.
King Mrgl-Mrgl is also appears in the Legion expansion. He helps a child murloc named train in Highmountain.
In Wrath of the Lich King
King Mrgl-Mrgl is involved in the following quests: