In Xanathar's Guide to Everything a new subclass for Bards is introduced: the College of Glamour. This subclass has a new ability called "Mantle of Inspiration", description follows:
At 3rd level, as a bonus action, you can expend one use of your Bardic Inspiration to grant yourself a wondrous appearance. When you do so, choose a number of creatures you can see and that can see you within 60 feet of you, up to a number equal to your Charisma modifier (minimum of one). Each of them gains 5 temporary hit points. When a creature gains these temporary hit points, it can immediately use its reaction to move up to its speed, without provoking opportunity attacks.
The number of temporary hit points increases when you reach certain levels in this class, increasing to 8 at 5th level, 11 at 10th level, and 14 at 15th level.
I have been running a Glamour(ous) bard in my level 5 campaign, and there seems to be balance issues with this ability. As a bonus action, for 5 rounds, the other 4 PCs in my party get 8 temp HP, and have the option of moving around the battlefield for the low cost of their reaction.
To me, the temporary HP amount seems unbalanced. Each turn (again, as a bonus action), I am able to dole out 8 temp HP. This is about 15-25% of our current PC health pools. I understand that characters must choose one temp HP source at a time, but our combat scenarios usually result in most members of the party taking damage each round anyway(due to ranged abilities and fast-moving opponents).
To me this almost completely overshadows normal Bardic inspiration, and thus lowers the interesting choices my character must make. Am I overstating/overvaluing Mantle of Inspiration?
Best Answer
A few things impact this...
You're meant to use this a lot.
Your Mantle of Inspiration is a parallel to a Lore Bard's cutting words, a Valor Bard's Combat Inspiration, and so on. This is your sub-class's 'fancy trick' with your Bardic Inspiration, and every sub-class gets a LOT of use out of their trick.
Having played a Lore Bard, I can freely say that I tended to get more use out of my Cutting Words than I did out of normal expenditures of Bardic Inspiration. The ability to reduce the total damage of AoE spells is fantastic, or make that dragon whiff when it takes a swing at my nearly-dead ally.
Burning your Reaction is not necessarily a 'low-cost,' and becomes less so as you level up.
Burning your Reaction means no opportunity attacks and it deprives several classes of special features they have (like a Battlemaster's Parry, or an Arcane Caster's Shield)
You seem to be getting an inordinately large number of Short Rests.
A Short Rest is a full hour of rest. The standard adventuring day as proscribed by the DMG is six to eight encounters, with two short rests interspersed in there. You should not have all 5 uses of your Bardic Inspiration for every combat.
Excessive Short Rests tends to screw with the balance of 5E (such as making Warlocks ridiculously powerful)
There are many things Mantle of Inspiration doesn't help against.
8 temporary HP isn't worth crap if a character gets hit with Hold Person and gets assailed with a flurry of auto-crits. But BI to boost their Saving Throw may save them from failing that save in the first place. It's better, mathematically, to successfully save vs. Fireball than to have 8 temp hp to help tank the damage. Temp HP is useless for aiding skill checks. And temp HP is only so useful if your party is having a devil of a time hitting their target.
In a typical combat encounter...8 temp HP and the chance to relocate for a Reaction is pretty powerful. But normal use of Bardic Inspiration gives your fellow players an "Oh Crap" fallback. It can let PCs succeed when they might not otherwise (such as letting the poor noisy Fighter succeed on a Stealth check).
In short...Mantle of Inspiration does precisely one thing, and does it well. This is definitely powerful in combat. But traditional Bardic Inspiration is the ultimate multitool. Need to succeed on a skill check? Need to hit with that attack? Need to succeed on a Save? Need to break a grapple before the Grell drops you off a cliff? There ya go.
Finally, Bardic Inspiration is something you can pre-load allies with. Once inspired, they can opt to use it at any time in the next 10 minutes. So if you're about to go into a boss fight, it's not a bad idea to go ahead and pass out Inspiration, leaving you clear to use your Bonus Action for other things and granting your party each one 'probably get out of trouble free' card..
Summary
If you are doing nothing but relatively straightforward combat against enemies that aren't packing Save or Suck abilities or high-damage save-based spells, versus enemies your party can reliably hit...then yes, Mantle of Inspiration will probably get used a lot. You shouldn't be able to use it as often as you do (see the bit on a your surfeit of Short Rests), but it's a good feature.
But MoI is nearly useless outside of combat and is mathematically inferior once you start dealing with most abilities that call for Saving Throws. Especially the ones that burden a character with debuffs.