Lowallyn was one of the three Numena of Dominaria. The others were Kuberr, god of the Cabal, and Averru. Lowallyn is the "Lord of Hidden Waters". He slew the blue Primeval, Dromar the Banisher. By doing so, he received Dromar's powers.
Lowallyn was later reincarnated as the illusionist Ixidor.
Scourge continuity issue
Lowallyn, along with several other popular characters, is part of a heavily-debated continuity issue concerning the Scourge novel. While Karona summoned Lowallyn, Fiers, Multani, Teferi and Yawgmoth, that meeting was later retconned without further explanation.