Contains spoilers, obviously.

If Nova is attacked: if a party member or small number of party members makes a rogue attack on Nova, she will attempt to flee or, if escape is not tactically feasible, to fight them without lethal force.

Gabrio: Unfortunately, Gabrio has magic-resisting gloves and flies, making it harder for her to deal with him; if she isn't wearing her horn-protecting armor she may take a harder-line tack than the others. If she can fly she can leap into the air and use pegasus magic to conjure concealing fog. If teleportation is possible she can teleport away.

Akasha: Attack is likely to be occur unexpectedly if it happens but will probably be more strategic. Fortunately she can't fly so Nova should just be able to get away.

Emerald: Unless Emerald is clearly only trying to subdue her (in which case she'll just flee if she can), Nova will tactically withdraw immediately, then return and respond with lethal force as soon as she has cast what buffs she can, because of the threat level Emerald represents.

If the party makes a deliberate attack resulting from a consensus decision, which is clearly coordinated and not a rogue attack, see per Emerald above. She does not want the party to have time to come up with a better plan than straight up attacking her, which is a comparatively bad plan. Nova will not be willing to take Emerald, Akasha, Qotho or Shadow prisoner; Gabrio (if he gives his word), or Discolor could be taken prisoner if they surrender. Though the game would still be screwed.

If Nova is disabled with a sneak attack or is killed: If her horn is broken suddenly, or a mortal injury is inflicted on her: the bonds holding her chimerical soul together break, releasing Eldritch Lore's soul fragment, who will fight the party with all available force.

If Risen Bones is attacked prematurely: If the subject of an impulsive attack, EL will roll with it and let the party "defeat" him.

A grating, high-pitched voice is emitted from the lich: Foolish mortals! You could have ruled by my side, joining us in the conquest of eternity! But now, only the void awaits you - prepare to die!

Nova will not aid Risen bones (for one thing, she'll be distracted pupeteering him). At some point:

Risen Bones: Don't just stand there, destroy them!

Nova will let the party defeat him. Before he is "destroyed": Risen Bones: Please... help me...

Nova will be legitimately sad at her little psychomachia, since she really is responsible for the death of her former friend and still has conflicted emotions about it.

Battle with the Hollow Spectre (Eldritch Lore's shade)

This is what will happen if Nova is killed or crippled; baring the party screwing with her using psychic magic deliberately, it shouldn't come up. But just in case.

You see a terrible apparition floating above you, towering at least 20 meters high. It has the form of a mare, with perforated fungal wings and crown of nine crooked horns. The spectre appears to be hollow and missing pieces of its form, denying it any kind of symmetry below its waist. Instead of hindlegs, a skirt of flowing chains hang down, each ending with a collar around the neck of some other shade. Their forms are many and strange - you have never seen the like of such creatures as you see before you, writhing in their ethereal bonds. One of the shades is unlike the others, resembling a small alicorn filly.

Hollow Spectre: ... I'm free! Yes... hah! You have released me from that... prison of conscience. Tremble, mortals, before the Undying Genius of Eldritch Lore!

Each of the sub-spectres has a different sort of attack, and a variable number of them attack each turn.

HS: Yes, fight me! Amuse your new goddess!

If she appears to be winning: HS: Pathetic! I expected better from you -- that _child_ must have been holding back to allow herself to be defeated by _you_!

If she appears to be losing: HS: Your attack _will_ come to naught! Not even the greatest of the Dread Pantheon could destroy me, what chance do you think you have?

HS can be defeated by the combined purified EoH or by gods; nothing else can really hurt her. If her bound spirits are all destroyed (except Nova/Elite Librarian's, which she'll just release), HS will just leave. This will also happen if she sees the assembled EoH but the party is slow on the trigger or does not have them all purified: HS: ... This -- no... Fine, claim the crown of Equus for yourselves if you wish... I need only wait.

If defeated with the EoH: HS: No! But I'm immortal, I --

The spectre is turned to stone, and falls to the ground, breaking into pieces.

Battle with Nova (fighting to win)

She will use a combination of earth pony, unicorn and pegasus magic. She uses her unicorn magic mostly defensively and for utility / healing, rather than direct damage, preferring to beat the stuffing out of opponents in melee. Because Gabrio flies, and threatens her in melee, and has magic resistance, though, she will probably try to kill him with an AOE attack, which she would target to hit others as well if possible.

A hit from her in melee (flying charge) would severely to mortally wound any of the party except Gabrio.

If Nova is attacked on the Harmony in an appropriate place, she might blow the party into space if the situation seems desperate, e.g. by attacking a window. Hope they drank their space mead.