Syntax: counterspell <target> <spell name>
You can counter another character or NPC's spell with one of your own. Your
spell must be memorised. It must be the same spell as that being cast by your
opponent or the dispel magic spell.
For instance if a character is casting fireball at you, and with the spellcraft
skill you have determined this is what is being cast at you, you can use the
counterspell ability to cast fireball, and your fireball will negate their
fireball and neither spell will take effect.