Re: any "summon earthpower" type spell for death magic?
 

Horror marking is best done with massed targetting on important units, like thugs, mages and other commanders.

Hence astral geyser is not a great choice. It's got mediocre accuracy so you're not likely to hit the right target, and in my experience it targets groups of basic troops anyway. It's mostly useful for the 2AN,MR area damage: although it doesn't do much damage, the range is good and fatigue cost low. If you have blood as well, you could fire astral geysers and then summon horrors, which will preferentially attack the horror marked units.

Re: any "summon earthpower" type spell for death magic?
 

An oft-overlooked side benefit to Strength of Gaia: its base fatigue cost is lower than Personal Regeneration. Their fatigue costs will be identical for a 3N caster, and SoG takes less fatigue at higher caster levels levels.

Therefore, if you have the paths to cast SoG, you can think of it as "Improved Personal Regen": it is better in all ways, by being cheaper and adding extra bonuses. Only downside is from foes with Fire magic :)

Re: any "summon earthpower" type spell for death magic?
 

I believe that Phoenix Power was skipped. It boosts fire magic and grants 50% fire resist. As far as I know it isn't restricted by the environment. Though I must admit I've never tried to cast it under water.

Re: any "summon earthpower" type spell for death magic?
 

Didn't phoenix power give 100% fire resist? Fire shield did, but I was under the impression you got full resistance from phoenix power as well.

Re: any "summon earthpower" type spell for death magic?
 

Just tested it again and it indeed is only 50%.