I like the left side of the tree a lot more than the right.
Horror is okay.. I actually think it's better to not upgrade it (the extra damage is really negligible) because then you can scatter a crowd and still hit someone with Full Draw at max damage. I usually focus all my attacks on one thing at a time so this makes a difference. It can be a clutch disabler against enemies that are seemingly immune to everything else. Nice that it's the first spell in the tree too.
Death Siphon isn't amazing but it's kind of nice to have because it allows you to use less Health potions. Sometimes you'll take an arrow or something but then you can finish the encounter with full health again just by continuing to kill off the trouble. The mana boost feels to me unnoticeable though I'm sure it's helped somewhere along the way. It's a nice ability to have, though if I didn't have to pick it up to go deeper in the tree then I might not take it.
Haste is okay but it seems really short everytime I use it. Maybe Dragon Age 2 spoiled me lol.
Power of the Dead I could care less about. I'm more about trying to keep my party alive through controlling the Battlefield. I would avoid this one when possible if I could because I'd much rather do something else.
Spirit Mark is usually completely unnecessary, but I actually like to take it. It can sometimes be amazing, though it's usually not. Since we're mana capped, 50 mana versus 65 mana makes a huge difference to me though when comparing it against Walking Bomb. I'll use Spirit Mark on bigger stuff, so the damage over time can factor in, but if you're using it for strictly damage then there are much better spells out there. It is however, very cool, and depending on what you mark, it actually can be amazing.... though it's almost always not. If you can afford a slot then it's a fun one to have, hence why I take it. Also be aware that you lose XP by using this ability. For some reason you don't get XP for the kill, it just straight becomes an ally. You can make it up by fighting extra stuff but it's just good to be armed with more information.
The other side of the tree I really don't like.
Blinding Terror feels pretty worthless to me since as soon as you hit the target they aren't panicked anymore lol. It's also going to be running so it's not the easiest thing to hit with every single high damage ability to use the bonus. I would avoid this ability as much as possible.
Simulacrum sounds good, but it's really not. Especially with the Revival combo mentioned by the original poster. If your party is going down that much then you have a lot of other issues going on. Instead use those skills on stuff that will keep you alive, such a disabling spells. Even a temporary disable can give you the time to kill something and let some money spell to cooldown all the way. I would take it over Power of the Dead if I could (since I like to take Spirit Mark as it's fun), but it's on the wrong side of the tree so.. I would consider it maybe at a really high level or something. It can definitely save a party in bad shape but the main point is that your party shouldn't be going down so much that this ability should be considered a big deal, and using that many ability points to get there kind of is saying just that. If you have extra ability points then hey this is a nice one for back-up.
Walking Bomb is just not good. If you're looking for damage then there are so many other better ways to do it, plus it's expensive in both mana and ability points. Casting this really limits what you can do next which is a lot considering that it's not a disabling spell either, so you still could have a wave of stuff after you. The explosion is bad too because you have to sit there and manage it, plus it removes the damage over time. If you could detonate the explosion and do the damage over time, I would like it a lot more but even still it has many issues. Can I envision a scenario where you detonated it in a group of mobs, then have it spread to the group, thus killing a whole group? Sure, but that's really not very likely and also a pain in the ass to manage. Can this one shot a grip of mooks on casual or something? Because I can't see it being very frequent at all on nightmare. Oh and I play with friendly fire on, so I have to be doubly careful not to detonate on my tank. Downsides far outweigh the benefits, so it's a no for me.
Anyway, probably the best ability here is Horror and even then that can be made up by other trees. Haste would be the best but it's a focus ability.. If you are good enough to take more fun abilities and still not have issues then this can be a nice tree, though it definitely has issues.