"Alēctō" is the name of one of the Furies, made surprisingly famous in the Harry Potter books. It seems to come straightforwardly from Greek ă- "not" + lēg- "stop" + -tos "[adjective]", so "unstoppable" or "unstopping".
However, Vergil refers to this Fury as "Allēctō", with an extra L. For example, see Aeneid X.41:
Allēctō mediās Italum bacchāta per urbes
Allecto, running wild throughout all the cities of Italy
Is there any linguistic or historical reason for this extra "L"? It certainly makes the name easier to fit into a hexameter, but Vergil could just as well have lengthened the vowel, like we see in I.2 (where he has Ītaliam for Italiam). And I'd expect Vergil of all people to know his Greek backward and forward, given his love of Homer.
EDIT: As sumelic points out, if my etymology is correct, the a in Alēctō would be short by nature.