The Bloody Siphoning Aura
This is an article with more details on what the in-combat aura does when you perform music in battle.

You can reply to this article with suggestions, feedback on the general usability, usefulness (or not) of the aura. I reserve the right to invoke mod-author privilege, in that a suggestion does not mean I'll work on it, but if there is a consensus or popular interest in something that is within my skills to create, I'll consider it for an update. If no suggestions, that's fine too - in which case I will default to working on whatever it is I want to see in the game for myself!

I'll add a quick caveat at this point here which is my mods are generally more oriented toward flavour stuff: visuals, music, whimsy, QoL, the occasional bit of silliness, basically tinkering with things to the side of core gameplay. There are lots of modders out there who are very passionate about spells, progression, balancing, enhancing the combat experience etc., so if that is your main or only interest you may not find exactly what you're looking for in this mod.

Bloody Siphoning
(Activate this by performing music in combat with one of the violins)

Starting a combat performance (Bloody Siphoning I)

• The lead performer starts music, and is joined by instrument summons to complete the song. 
• Concentration is used to maintain the music performance, and the performer cannot take actions while focused.
  

• The entity places Blood Sanctuary on the lead performer (basically just sanctuary, to assist with keeping performance on)
• All nearby allies receive +1 AC, +1 to attack rolls
• All nearby enemies receive the Haunting Harmonies status (a chance of 1d4 psychic damage per turn and the Off Balanced status)
• The lead performer receives -1 constitution
  

• All nearby allies receive an additional +1 AC, +1 to attack rolls, and the roll required to crit is reduced by 1
• The lead performer receives a further -1 constitution
  

• All nearby allies receive an additional +1 AC, +1 to attack rolls, and the roll required to crit is reduced by a further 1
• In total by the third round the boosts are now +3 AC, +3 attack rolls, crit threshold reduced by 2 and enemies Off Balanced.
• The lead performer receives a further -1 constitution
• Two spells are unlocked for the lead performer on the third round: 
• Expel Bloody Siphoning which is a controlled way to remove the siphoning aura boosts and break music performance concentration. Deals damage aligned with Lightning Bolt (more or less - if you're on version 2 of the mod the damage will scale with character level).
• Bloody Eruption probably don't want to actually cast this unless you have a tactical or positional advantage in the knock back effects. But this is an indiscriminate "losing control" spell that does damage to the lead performer (slashing, 6d6), and knocks back everything nearby. The performer will receive two turns of Chest Trauma.
• The lead performer can either choose to expel Bloody Siphoning in a controlled way, or sustain the music performance and keep the ally boosts and enemy debuffs up. 
• From this round onwards, there is a 40% chance at the end of every turn Bloody Siphoning is sustained that Bloody Eruption is triggered automatically.