The creeping embodiments of nightmares

Pugtastic has some more information about these devilish beings.

Crithto: The creeping embodiments of nightmares,

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Twilight Fiendlings can be found in corrupted areas of Twilight Highlands—though you may need to quest a bit in the region to remove the veil that hides these pets from newcomers.

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(In other words, they may be affected by phasing.)