How to use egg bombs？
To add to what Greenstar said:
There's two kinds of egg bomb, both work the same but one is a "Curse" card draw, the other is what they described:
Part of the 'Green Cards' that you always have access to (Pitons, Torches), for mouse and keyboard you scroll the mouse wheel while hovering over them, and one of them will be any egg bombs generated by transformation.
The curse ones are "Provided" by the Harpies directly, and you have to draw them like other cards. You have to fulfill a specific condition to have it happen, but there's hints in the gallery.
Once you have them, drop it on yourself to put the egg in the MC's hands, then you pick one of the three pink eggs in the 3 spaces under the MC to 'Aim' the egg in one of those directions. It will then begin moving in that direction, and accelerating if it doesn't immediately collide with anything.
Additionally, there's a funny quirk of how it's deployed. Since the egg 'deploys' right on top of you, you can click yourself to 'push' it like when an enemy overlaps with you, to shove the egg either to the left or right, or if you aimed it straight down, you push it back up into the air...
Which then causes it to move down onto you again, letting you push it back up a 2nd time. It accelerates to moving 2 tiles at a time, so you can't 'stall' it a third time, but it can sort of kickstart the damage, AoE, and speed of it, if you have time to set it up, but couldn't get higher in the air to do it naturally.