Your team spirit has given you an extra hand, as long as you have at least one of your allies within 5 feet of the creature you're attacking, and the ally isn't [condition]unconscious{/condition]. You can now gain a plus to your attack rolls equal to your proficiency bonus, you have to decide rather or not you use this before you make the roll. You can do this a number of times equal to your proficiency bonus amount of times per long rest.
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