The Sceadugengan (Singular: Sceadugenga), or "shadow-goer", are fantastical beasts which are neither living nor dead, and which can shape-shift. They dwell in the forests of England. In some tales they often take the form of children, acting likecuckoos by moving into a village and settling down with a compassionate family. They stay to get both food and companionship as they are solitary for the majority of their life. When they feel like the attachment is too much or when they become bored they often leave to return to the forest. However, they will stay with humans indefinitely if they fall in love with or form an attachment to a human. Over time, they lose their abilities and forget what they once were, living out the remainder of theirexistenceas a human.If a human became suspicious that the child was a Sceadugenga they need only look at their eyes as the irises were always a pure black with no pigmentation. The first element of the name is from Old English sceaduwe (shadow) and the second elements is likely to be from Old English gong (a going or journey) or Old English gan (to go).
The Sceadugengan are mentioned in Beowulf and from part of a sub plot in The Saxon Stories series by Bernard Cornwell along with various orally passed down tales.
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This is an excerpt from this link: Viking Answer Lady

Like the draugr, Grendel is a creature of the night. He is deogol dædhata deorcum nihtum (l. 2775, "the hidden doer of hateful deeds in the dark night"), the scriðan sceadugenga (l. 703, "the gliding shadow-goer"), who moves through the landscape of shifting shadows and intermittent moonlight so characteristic of the undead: Sceaduhelma gesceapu scriðan cwoman wan under wolcnum (ll. 650-651a, "the shadow-cloaked shape comes gliding, black under the clouds.") Grendel conceals his actions beneath a mist, Ða com of more under mistleoþum Grendel gongan (ll. 710-711a, "Then he came from the moor, under hills of mist Grendel went"), and the intermittent moonlight suggested by the clouds and mist may be responsible for the gleam of light from Grendel's eyes: him of eagum stod ligge gelicost leoht un fæger (ll. 726b-727, "from his eyes came an ugly light, most like a flame").