ToME: the Tales of Maj'Eyal

All right, I've stared at this tooltip for way too long.

It's an instant cast shield for low-mana situations. It's got a shielding component, a mana recovery component, and an explode and kill you component. I really am at a loss about how to juggle these aspects to use the spell for maximum benefit.

Since it's instant, I suppose I could set it to "auto-cast when available", and not have to consciously think about it (similar to setting Time Shield and Runes of Shielding to "auto-cast when an enemy is visible"), which would make Disruption Shield automatically trigger whenever my mana is low enough. Is this what people typically do with it?

It is actually not very common to die to your own disruption shield, especially if you don't stack arcane damage, because its explosions both have minimal damage and give you a lot of arcane resist.
When you're running out of mana, the fight is probably taking longer than it should, and you want to make conscious decision about how to spend your turns instead of relying on auto-casts. Shielding runes are instant! But when you're low on mana, Manasurge is quite often a better option, especially if you still have shields around you.

The tooltip is pretty confusing I will admit. Just to clarify the order in which shields are damaged: Temporal, Damage, Displacement, Disruption, and then HP. Assuming the first 3 are down or get destroyed and you take 1 damage, instead of that being subtracted from your HP it instead gives you X.XX mana (whatever it says on the tooltip, it caps at 0.15). It will then check if you have full mana, if you do the disruption shield will break and will do X damage per turn which is equivalent to however much damage you have absorbed, capped at X, divided by whatever, giving you very high arcane resistance so it doesn't kill you. It will absorb damage infinitely, so long as you don't get hit for enough damage to bring yourself to full mana.

So with this in mind, the best thing to do is simply keep the disruption shield always on, which doesn't require autocasting etc. You can also activate Aegis which gives you around twice your max mana in empty mana. Using the shield explosion as a damage dealer is not really a smart idea as you are forgoing a significant amount of survivability for relatively little damage.

There's not really much benefit to just keeping it around for low mana situations, if you manage it properly, by putting another shield up before your mana gets too high you can simply fluctuate your mana and always remain at around 80% instead of sitting at 0% and not doing anything as your mana recharges.

Hope that helps!

That helps a great deal. Thank you both.

So basically, to make sure I get this, Disruption Shield is essentially using the empty part of your mana bar as a shield with no duration limit? Or to put it differently: never take HP damage again unless you screw up and accidentally end up with full mana?

Definitely going to give that a second look, with a more deliberate approach to managing shield/mana levels.

That's basically it, plus don't forget the tip of firing off Aegis if you get too full; it'll raise your maximum mana level by a hefty percentage, allowing you to safely drain off the excess before the shield collapses. The damage it does if it does give way is pretty trivial. It shouldn't kill you as you are likely to be doing pretty well inside your bubble there, what will all incoming damage having been rerouted to your mana pool for some duration, and the arcane resist it gives really further reduces what punch it has.

Shields are really the key to the archmage, in my opinion. People get mired in a "glass cannon" stereotype and decline to emphasis a key strength that makes it a rather durable class, all in all.