Part 262: Extras - Blood Magic

Blood spells worth using or at the very least that people will complain about. Things I think are /really/ worth casting I put in bold.

Level 1:
Summon imp - It's a nice combat spell that will pretty much never kill anything unless darkness is up, but it can be used to delay scripting or mess up AI targeting. As thedemon said Swarm is better (but this requires less research).
Bind Bone Fiend - These are very cheap chaff that are basically your best units ever for absorbing spells. Low cost, high hp, decent amount summoned.
Bind Spine Devil - These guys are similar to bone fiends, but they can also deal out decent punishment in melee. However, you get only one instead of 3, and still cost 3 slaves. So its a much higher investment in terms of mage time and slaves.

Level 2:
Bowl of Blood - Its a site searching spell!

Level 3:
Bind Devil - Heat aura, good weapon, and flying. They also require 7 slaves a piece and a B2F2 caster, so they're not easy to summon.

Level 4:
Blood Lust - Battlefield +4 strength to demons.
Bind Storm Demon - They're basically the best troop from blood summons, only beaten in some niche situations by demon knights in my opinion. Also 7 slaves and A2B2.
Bind Demon Knight - Fear, decent survivability, and a lance. 5 slaves and E2B2.
Rain of Toads - Probably one of the most complained about blood spells, second only to AC. Causes 40 unrest, +2 towards the misfortune scale, and a 5% chance to disease all units in the province (5% chance per unit).

Situationally call lesser horror is also helpful, but difficult to really use well in a battle. Call horror is a better bet.

Level 5:
Hellbind Heart - It's an enslave mind/charm spell. However, unlike those two it costs slaves and has 100 fatigue, so its much, much harder to spam.
Horde from Hell - This is only useful because in darkness situations it can take on almost any PD. (remote attack spell)
Send Lesser Horror - This is a better bet for remote attack spells. Sends a lesser horror, one of which can probably take on up to 15 points of most pd, maybe even more. Reverts the province to neutral afterwards.

Level 6:
Rejuvenate - it takes 10 years off of a mage, its really only important if you have a mage who got a random that you really dont want dying of old age/disease.
Infernal Disease - It sends a disease demon to go assassinate a commander. Worse than an ashen angel and requires B5, but it only costs 5 slaves.
Call Horror - If you can actually muster some B4S3 mages, this can be worth casting in battle. It only requires you to have B3S3, but if you don't have at least B4 your mages will fall unconscious and never recover and the horrors will eat them.

Level 7
Leech - Probably one of the best blood spells, and realistically the first really useful battle magic you get for blood. Its range 25, precision 100, 25+ AN damage. It is very useful and definitely worthwhile, but its blood 7 and one of the very, very few blood combat spells worth casting.
Blood Rain - This can situationally be kind of cool. It reduces all morale on the battlefield, so if you have a largely morale 30/50 army it can help route someone else's army.

Level 8
Send Horror - There's not many places where a horror will fix your problem and a lesser horror won't, but its nice to have when you do have something like that.
Rush of Strength - battlefield wide +4 strength for all your units.
Life for a life - Pretty much anything you'd want to cast this on you'd probably be better off casting leech on. However, I can think of one or two situations I'd use this over leech. The biggest thing it has on leech is having 100 range. 199 fatigue, B3, range 100, 40+ AN damage.

Level 9
Infernal forces/tempest/crusade (B5 E2/A2/F2) - You pretty much cast these because it saves you some mage time. It's really primarily helpful for nations like MA Abysia who only have cap mages capable of summoning the demons/fiends.
Infernal Prison/Claws of Kokytos - These are probably bitched about as much as rain of toads and astral corruption. It sends a unit to Kokytos or the Inferno taking them out of commission for a while. There's no way to resist it so it's complained about a lot. Truthfully though, anywhere this might be helpful, you probably could have just done the job with leech.

OPINION: Level 8 and 9 blood aren't worth researching. The amount of return you get for actually investing your RP into the school just isn't worth it. Almost everything is situational or can be accomplished just as well by lower level blood spells. Blood has comparatively little helpful battle magic relative to other paths, and it's SCs and thugs, especially with EDM summons, are very lacking.

In my opinion, there are two things that blood does remarkably better than other paths. Having troops summons worth casting, and it's ability to kill single targets. Most other paths have useful ways to kill single target units, but most of bloods (particularly leech) are irresistable and that's a big deal. Additionally, every other path really stops getting troop summons at level 6 in conjuration as everything is replaced with thugs/SCs. It's one of my biggest gripes about conjuration, and why I am now currently working on a spell overhaul mod.

Edit: For what it's worth, I think astral is the best path in the game, or at the very least it was my favorite/most used in vanilla and hard for me to break my love of it. It's arguably fallen out of favor since EDM/AE boost every other path significantly and kind of leave astral to rot.