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.
In a high canyon in the mountains, the players find a skeleton in a cage suspended from a pole. A few miles further, they find another, and a third contains a partially rotten corpse. The fourth contains a living man who looks as if he hasn't eaten in days. Turns out to be the local way of punishing criminals.