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the text of the spell astral horror says the material component is "a bit of spiritual essence".

what is it?
how can my nethermancer obtain some of it?
is it used when castin the spell, so i have to get a new one every time?
how can it be stored?

etc...

It's not a component, it's merely a description of how the adept achieves the effect.
He creates the illusion of a fierce spirit, but he does that by using a small amount of actual spirit that he conjures (but not a full spirit, that would be more difficult). Like having a bit of truth to facilitate a lie.

thanks. so there is not much in the way for a nethermancer to have one astral horror follow and protect him while exploring a kaer. or summon a lot of them every combat (if he has time for it).

No, it's only a time thing.
Out of combat, it's that it only lasts a few minutes.
In combat, he takes at least 2 rounds to cast the spell.
Well, and keep in mind that every attack against the things or an attempt to dodge their attacks is a sensing test already...

I still find it very powerful. im not saying too powerful however, but its disbelief difficulty is wil+6, which is about 18 using willforce for a level 5 nethermancer with 7 step willpower and 5 willforce. the sensing difficulty is about 31 then. I think thats practically unbeatable for a lot of opponents, for example equal circle mortals. am i right?

It's about 3% for a straight-up test (just attribute + rank), which would apply to sensing by Avoid Blow or regular attacks. This would cover non-adepts. But if you have adepts, and factor in another hypothetical +rank that might result from using a buff-talent such as Anticipate Blow, and like a +3 to +5 bonus from situation or Parry or magic items or beneficial spells, and maybe Karma, you're quickly getting to 7-10% (the numbers are crooked, but a Step 20 would have an 11% chance of success against a 31). Given that you'll have multiple chances at it, and that it's only a side-effect of the attack, it's "beatable enough" for an adept, IMO. If this thing seems enough of a physical threat to you to go all out and get all these bonuses, you're getting a good chance to actually disbelieve.

thanks!

i nearly always use my astral horrors to attack agressively, so they are very much a physical threat.