This spell can be cast on two creatures. The Cleric affects creatures that are within 5 feet of each other at the time of casting. After the spell is completed, affected characters may move about freely. Defensive harmony grants affected creatures a defensive bonus by bestowing an enhanced coordination of their attacks and defenses. The affected creatures must be involved in a single battle and fighting one creature so that their efforts harmonize to the benefit of all involved. If they split up and fight separate enemies the spell ends. While the spell is in effect, each affected creature gains a +1 bonus to armor class for every other creature benefitting from the spell. So at 4th level spell slot, if two people are fighting one creature, they get a +1 bonus to there AC. The spells last 1+2D4 rounds
At a higher level spell slot you can target more people for this bonus. A 5th level slot, you can target 3 people which would boost the AC bonus up to +2 to AC
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