College of Battle Songs

Within the realm of bards lies the specialized path known as the College of Battle Songs, where the wielders of this subclass harness the mystical potency of music amid the chaos of battle. Proficiently attuned to their chosen instrument, these bards orchestrate powerful magical effects through four distinct musical modes: the uplifting resonance of "Major," the haunting echoes of "Minor," the enigmatic harmonies of "Mystic," and the radiant crescendos of "Bright." In combat, they effortlessly weave their melodies, invoking spells that sway the tides of battle in their favor. Their mastery in the art of musical manipulation grants them the ability to bolster allies or afflict adversaries with discordant notes that disrupt and hinder. The Bard from the College of Battle Songs, almost exclusively dedicated to the intricacies of battle, serves as a testament to the sheer might and versatility of the harmonious enchantments attainable by dedicated practitioners of the musical arts.

Master of Songs

When you join the College of Battle Songs at 3rd level, you gain proficiency in the Performance ability if you do not already have it. Additionally, when you make a Performance (CHA) check  to play an instrument that you are proficient with, you make the check with advantage.

Additionally, your Song of Rest feature changes into Song of Reinvigoration. Instead of granting extra healing on the use of a hit die during a short rest, you can give temporary hitpoints equal to your Bard level to all allies within 30 feet that can hear you as an action.

You also gain resistance to thunder damage. This increases to immunity at Bard level 11.

Thunder Immunity

You gain Immunity to thunder damage.

Modal Inspiration

When you use your Bardic Inspiration feature, you may choose between four modes for your accompanying performance: Major, Minor, Phrygian, and Lydian.

A creature that has a Bardic Inspiration die from you can roll that die and use it in four different ways depending on it's mode.
Major: As a normal Bardic Inspiration die.
Minor: As a reaction to being targeted by a spell attack or melee attack to subtract the number rolled from the attack.
Phrygian: To cause their next spell attack to critically hit on an attack roll of 19. This does not have to be declared before the attack is made.
Lydian: As a reaction to taking damage to heal for the number rolled.

Improved Countercharm

In addition to the normal effects of Countercharm, gain added effects based on the mode you're currently playing in.

Major: Countercharm also gives advantage on all Wisdom, Intelligence, and Charisma saving throws to all allies within range that can hear you until the beginning of your next turn.
Minor: Countercharm also imposes disadvantage on all enemy saving throws within range that can hear you until the beginning of your next turn.
Phrygian: Countercharm deals thunder damage equal to your Bard level to a number of enemies equal to your proficiency bonus that are within range and can hear you.
Lydian: Countercharm heals all allies within range that can hear you for an amount equal to 1d8 plus your proficiency bonus plus your spellcasting ability modifier.

Accent

Starting at 6th level, your improvisation has improved to the point that your playing can effectively accompany the changing tides of battle at any given time. You accent a specific note in a phrase to produce different effects based on the mode you're currently playing in. When you use this feature, choose which of the following abilities corresponds to your current mode:

Major: As a reaction to an ally within 30 ft. that can hear you taking damage, grant them 10 ft. of movement that does not provoke attacks of opportunity that they can use immediately as a reaction.
Minor: As a reaction to an enemy within 30 ft. that can hear you being targeted by a spell, slow their movement by 10 ft. and prevent them from taking reactions until the end of their next turn. (This happens after the effects of the spell are resolved)
Phrygian: As a reaction to an enemy within 30 ft. that can hear you taking damage, deal thunder damage to that enemy equal to your Bard level.
Lydian: As a reaction to an ally within 30 ft. that can hear you being healed, grant that ally a +2 bonus to their AC until the end of your next turn.

Ascension

At the end of every round in combat, your music rises in intensity along with your current mode of playing and echoes out a magical blast.
Choose from the following four abilities according to the mode you're currently playing in:

Major: All allies within 120 ft. that can hear you gain +1 AC and 30 ft. of extra movement until the end of the round.
Minor: A number of enemies up to your spell casting ability modifier within 120 ft. that can hear you must make Charisma saving throws equal to your spell save DC. Whenever a target that fails this saving throw makes an attack roll before the end of their next turn, the target must roll a d8 and subtract the number rolled from the attack roll.
Phrygian: A number of enemies of your choice up to your proficiency within 30 ft. of you that can hear you take thunder damage equal to your Bardic Inspiration die.
Lydian: All allies within 30 ft. of you that can hear you gain a number of hitpoints equal to your Charisma modifier.