Normally, spells receive bonuses from (1) the category that's required to learn it, and (2) sometimes additional bonus from an attribute, e.g. Fireball receives bonus from Pyro and Intelligence.
It requires Necro 2, so supposedly it should scale with the Necro stat, and also, the bottom part of the description says "High intelligence improves this skill", so it should also scale with Int.
But it doesn't seem to scale with either one!
Fane: Necro=0, Int=5, Bone Cage=632 phys armor Ifan: Necro=0, Int=12, Bone Cage=775 phys armor Sebille: Necro=5, Int=23, Bone Cage=574 phys armor
Then, I also lowered Sebille's Necro and Intelligence stats by removing her gear, and it made NO difference in the Bone Cage armor bonus, which confirms that it doesn't scale with either one.
Armor/Hitpoints might have a influence too? Sebille has the highest amount of armor and HP and therefore would receive the least abount of armor?
Also might be bugged, would not be a first in this game.
Edit: On a second look I think Geomancer makes up for the differences, Geomancer affects armor gain in general if I remember correctly. Though the description is still buggy, because it says, Int would have influence.
Kalrakh, you are correct about the Geomancer dependency. I realized that after I made this post.
And this is one of my biggest frustrations with this game - the misleading, confusing descriptions, and in this case outright lying to the player.
To Larian:
This is too difficult to keep track of, especially for someone like me who plays the game only once in a while. I can't possibly keep an encyclopedic knowledge of the game mechanics and all the stat dependencies, especially those that don't make logical sense and are hidden from me - e.g. here you have a spell that requires points in one magic category to simply activate it, while the actual spell performance depends on a completely different magic category (which makes little sense, and is not mentioned in the tooltip at all!), while the spell description incorrectly states that it depends on a totally different attribute altogether. I fail to see the upside of having descriptions that are so cryptic and sometimes plain wrong!
In effect, Bone Cage is really a Geomancer spell through and through, not a Necro spell, because the added physical armor depends ONLY on Geo, which is the most important factor here. Just because you made the spell requirement two points in Necro doesn't justify calling it a Necro spell at all. But in the entire spell description there is absolutely no mention of the word "Geomancer", which is what makes it work. That's a problem.
Every skill tooltip should have a "dependencies" section that makes it clear which stats affect the skill performance. That's not too much to ask, since many spells already provide information like "receives bonus from Intelligence", etc. You just need to be consistent and list ALL dependencies for ALL spells, so the player doesn't lose their mind trying to figure out nonsensical values, and hidden information.
p.s. The combat and skill system is already complex enough (mostly in a good way), so we really don't need the additional 'difficulty' of dealing with cryptic and misleading text.
"Provides armor equal to 50% of Fortify and another 50% for each corpse in the area" ~ Fextralife website https://divinityoriginalsin2.wiki.fextralife.com/Bone+Cage