Poison

The Poison spell is meant to weaken enemies.

Spell Description
Casting the spell shoots a poison arrow at an enemy. If it hits, the enemy is weakened by the poison, losing much of his fighting power. However, many creatures are immune to this spell, and often enough the poison won't hit or have no effect at all.

The Green potions fulfill the same purpose as the spell.

Note that the spell Poison Wind is a much stronger version of this spell which can effect entire groups of foes, while the spell Poison Weapon does, what the name says.

Trivia

 * Since enemies can be force-fed green potions in Ultima VII, the spell is redundant in that game, especially since green potions are cheap to buy.