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I think it's obvious by now that certain spells and abilities are more equal than other spells and abilities, here's a hand tier list:
DISCLAIMER: There are spells that belong to a certain group that are however higher or lower rated because of certain quirks, stuff like Windblast (which is easily higher classed than other AoE spells) or Traitor Kin (which is lower than other direct damage spells because it's fairly weak). For simplicity's sake I've left them out for now.
T0 - No Brainer
Any direct damage spells/abilty + any healing spell/ability*
Defeating your enemy requires destroying the HP of his units. Of course spells and abilities that do this in the most direct way are of the greatest utility. Investing WoM into draining HP directly is propely the most cost-efficient trade in the game since you only spend a non-vital resource on taking something your opponent cannot easily regain (or at all regain). Likewise preventing your enemy from taking your unit's HP is just as important. If your opponent has exhausted and bruised his Dwarf Slayers in trying to kill your Carnosaur, being able to undo their effort with a simple click is another highly cost-efficient trade. Therefore direct damage and healing spells are of top tier utlity.
Examples: Spirit Leech, Fate of Bjuna, Liber Bubonicus, Soul Steal, Accusation, Melkoth's Mystifying Miasma (which is a slow-down too), Final Transmutation, Regrowth, Item-Bound Regeneration* (doesn't come with the 20% fire weakness), Earth Blood, Apotheosis
I don't count the VC healing here because the whole of the VC army is balanced for it with very low stats, so their healing is more of a necessity than a pure boon here
T1 - Highly Efficient
Snares and Slow-downs
Being able to impede the mobility of an enemy unit is of great use since that opens them up for all sorts of punishment, be it via units or via spells/abilities that require aiming. It also can negate a unit's charge bonus and save the bacon of your backline in a pinch. Snares and Slow-downs still require an additional element for maximum efficiency and are somewhat situational, so they're one step removed from T0.
Examples: Net of Amyntok, Tempest, Howling Warpgale, Warp Gaze, frostbite inducing attacks
T2 - Mostly Efficient
Stat Buffs and Debuffs
Any spell/ability that makes your opponent's troops fight worse and yours better requires the right timing and situation to cast it. A mistimed Pha's Protection or Soulblight might end up being completely wasted. Still, cast at the right time they can be devastating, so their usefulness is even more dependent on any given situation which requires more "work" and therefore lowers their utility tier. There are also some buffs/debuffs that are just overcosted or plain useless (Birona's Timewarp for example).
Examples: Cascading Fire Cloak, Flesh to Stone, Curse of the Midnight Wind, Curse of Years, Mantle of Khorok, Fists of Gork,
T3 - Sometimes Efficient
Magic Missiles and AoE/Wind damage spells/abilities
AoE damage spells almost always require your enemy to make major blunders and blob his troops up for maximum efficiency, which doesn't happen with any half-way competent opponent. While they can still do damage when cast on the right target, most of them have a fair forewarning period which means the opponent can try and dodge out of the way. Magic Missiles have been buffed a bit, but they suffer from having to set up a clear shot and at least for spells it makes more sense to use something that requires less time and effort to invest. Bound magic missiles and AoE abilities are a bit higher-rated because they don't cost WoM to use and are in a sense "freebies".
Examples: Dwellers Below, Plague, Piercing Bolts of Burning, Searing Doom, Cracks Call, Ruination of Cities, Fireball, Shem's Burning Gaze, Vindictive Glare, Gaze of Nagash, Viletide, Awakening of the Woods, Soul Quench, Plague Breath, Scorch, 'eadbutt, Brain Burster, Amber Spear
T4 - Inefficient
All of the moving vortices have the same problem, they have a >50% of having low, zero or even negative RoI, and that chance increases with every turn the vortex takes. Sure some of them can be quite devastating, a Banishment or Firestorm that moves exactly right can probably do the most damage out of anything in the game, but that's the catch, that perfect movement pattern is excessively unlikely to occur and most often any WoM invested in them are wasted or repay very little. Again, bound vortices are a bit higher rated because of their "freebie" status
Examples: Chain Lightning, Gehenna's Golden Hounds, Purple Sun of Xereous (somewhat increased utility because it sucks units towards it)