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.
The PCs find a small green glass bottle, something can be seen writhing within the smoked glass. If the PCs open the bottle a repulsing form of life will pour forth and grow in size. A genderless beings of soft pliable purple flesh towers before the PCs, it has no detail, no distinguishing characteristics except for a narrow slit for a mouth that holds rows of inwardly curving teeth.