An incarnation of Laserbeak, and condor/cyborgs for the Cosmic Era
Do not meddle in the affairs of wizards... -- Gildor, via J R R Tolkien
The first sign of the Army of the Lost are the legs. Blown off legs, hopping along wearing their military boots. And not long afterwards, the rest of the Army arrives, and the horizon from left to right is filled with the decomposing bodies, many of which carry weapons. Unless you can outrun them or fortify your position, you will soon be joining this army of the Undead.
A race of beings actually IS invunerable while in adolecence. They age, but cannot be killed. A miniority of the race (1 in 20) does not become invunerable, but rather becomes immortal at some point in thier adolecence, and ceases aging while being vunerable to death in all the normal ways. Another minority (also 1 in 20) Permanatly becomes invunerable after adolecence, but ages twice as fast as the race normally does. There is no known way to test for which of the three traits an individual manifests and the three traits cannot mix, as they are tied to the same gene, as it were.