This could either be 2 separate traps or a trap with 2 settings (like the skeleton trap.)
Trap/setting 1: Poison Trap. Area of effect similar to giant bomb. Creates a small poison spell (basically, although damage may need to be adjusted) that will dissipate after a set time.
Trap/setting 2: Acid Trap. Area of effect similar to giant bomb. Creates a puddle of acid that stays until the end of the battle, but can only damage ground-based troops.
If combined in one trap, call it a Chemical Trap with the two different settings.
