Casket of Fury

ZERO - at the beginning, it deals very good damage with its sword. Especially against archmages - one less enemy. The rockfall is not very strong (yet), and the area it affects is minimal. Now I'm getting spike upgrades, and gradually, this skill is transforming from experience gain into real power. "Wall" - I didn't understand the joke.
SLIIM - I really like Hellish Barrage now. At the beginning, after such an avalanche, the enemy ranks thin out. But in the middle of the battle, I prefer the sword: it deals more damage, and you can use the barrage later to hit your own units. The spit is too weak; when I get the barrage, I only use it for experience, as it costs less rage. Cloud? You're kidding, who will be harmed by 40 damage? But the "Gloat" is an interesting thing. Now, at the first level of upgrade, it's one attack against an enemy with no damage, but later, I think it will be invulnerability for one of the archers until the end of the game. And the archer can attack from under the "Gloat".
LINA and REAPER - I haven't unlocked them yet. But after reading the manual and player messages, I can't wait to see them. According to the manual, I especially liked the orb and "Gizmo" in Lina. And all of Reaper's skills look very nice, especially "Time Back" is interesting - resurrection and teleport in one. But 10% at the beginning and 50% for 90 rage at the end for "Soul Steal" look depressing. But "Black Hole" already impresses me with its numbers in the manual. It's just that it requires a lot of rage :(.

I'm not voting yet, as I haven't seen all the rage spirits. But even now, it seems to me that the spirits are effective in combinations: at the beginning, "Zerok + Sliim", and then "Lina + Reaper".

He supplements his previous post, based on new observations.

The first three spirits only have two good skills each. Zerok has a very good sword at the beginning, then you need to develop "stone spikes". Unfortunately, I took all four skills, so the "spikes" didn't get the proper development. Perhaps, if the damage from "spikes" approaches the figure of 600, Zerok will become usable in the middle of the game. But as it is - no.

Things are even worse with Sliim. At the beginning, his "hellish barrage" is really a good help. "Gloat" is not so useful; I think that even with good development, I will not use it often. Sliim is only useful at the beginning. His "cloud" and "spit" deal very poor damage. I know about "magic source" combined with "cloud", but this is not very useful for a warrior :(.

Lina is much more interesting. "Energizer" is not an option: it's expensive, it rests for a long time, it's unnecessary. But "ice orb" is something completely different. Two upgrades in damage and life - now we have a powerful unit. Then it would be very good to reduce rage as well. At the beginning, "Gizmo" is not very impressive. But 7 rage is not a lot of money. But later, with development, it even surpasses the orb! The most economical skill, the most adequate, in terms of price/quality. It needs to be upgraded in damage, healing, and 200 is quite enough. But extra damage is never superfluous :). In the end, "Gizmo" is still better.

Reaper is the most interesting rage spirit. He has the most adequate - "Soul Steal". Rage grows in geometric progression, it's not worth taking more than 40%, but with a reduction in cost and a reduction in rest by one, this skill is striking in its power: killing 500 fairies from 2000 is very good. As is 5 out of 25 demons. Class :):)! Rage generation. It has a huge drawback: after it, Reaper rests for 5 turns. But this can be fixed. The optimal option: generate 25 rage, 3 turns of rest. That is, first, slightly increase efficiency, and then reduce the rest time. "Time Back" hasn't impressed me yet. But there is a benefit to it. I know a cool tactic based on this skill, but that's not what we're talking about. It's just that this skill is also an additional "resurrection" :)! If you're lucky... And "Black Hole" is very relevant at the beginning of the battle: the computer divides its army into several stacks (I mean each unit). Each one gets hit hard :):). The bonus per level is also very good. But it costs a lot. You can't develop it much: even a warrior will start to have problems with rage. But even so, the damage is decent. The most optimal damage is 700-1100, but it is necessary to take one reduction in rage. 60 is a very good number for using this skill :)!

In total, the most useful spirit is Reaper. However, this is already clear from the voting :)!