Marian Leigh is a small city on the north banks of the Orpheid, in southern Canterwall, Ustalav. It's barely a day's ride from Gallowspire, or it would be barely a day's ride from Gallowspire were the terrain not profoundly unfriendly. The Hungry Mountains loom on the western horizon; the sun sets very early in the day. There is a fortress overlooking the river, and the city has forbidding walls in good repair.
The Whispering Tyrant's garrison here is commanded by a minor lich called Casavel, only sixth circle, not much of a threat in himself; they aren't expecting much trouble until the enemy starts Teleporting in, and are hoping they can take the fortress by then. The plan to take the fortress isn’t that complicated: Disjunction, have teams teleport in and flood the place with positive energy and slaughter everyone.
(Normal besieging of a fortress is for people with less magic than both parties to this conflict. They will get close under Mind Blank and Invisibility, Disintegrate some holes in the walls, and then fight.)
They’re putting up elaborate sets of protections, but Tar-Baphon will obviously open with a quickened Disjunction, once he arrives; the main thing they want to do is be dispersed enough he can’t usefully get a large share of the force that way. And of course Elie and Alfirin can counterspell him, though this is harder than it sounds. Tar-Baphon, like any self-respecting archmage with ambitions of divinity, puts up True Invisibility which Invisibility Purge and Glitterdust fail to reveal, unless cast as ninth circle spells, and Mind Blank, and then casts all of his spells from the sky at as much range as possible. They have to detect him casting to counter him. They’ll have people in the sky, of course, who can serve as sentinels of his arrival by how they’ll be instantly paralyzed and fall to the ground if he’s within a hundred feet or so of them, and he won’t be able to get a disjunction indoors unannounced, and if they have enough time they can dome the whole place in Walls of Force and force him to waste that opening Disjunction on those, but they probably won’t have time.
They know the layout of the fortress, from their scrying, and where there are magical traps that are dangerous enough to be worth dismantling rather than running through and healing afterwards. They know approximately the strength of its defense: it’s not trivial, but it will go down like most things do to massed Chain Lightning. They have some Mass Heal, the single best spell for hand to hand fighting with the undead, but they’re saving it for emergencies.
The hour of the attack is busy. They’ve gotten the long-lasting spells done in advance of the casters’s sleep - Mind Blank, Moment of Prescience, Greater Magic Weapon, Magic Vestment, Mage Armor, Arazni’s Heroism which when cast by Iomedae lasts twenty-four hours – so in the morning it’s the ones which will last a few hours, and then the ones which will last about forty minutes, and then the ones which will last just a few minutes. Almost everyone around here fights with a Bestow Grace of the Champion and Elie gets a few pitying looks for his not being Lawful Good. Iomedae does the Hastes; she knows Arazni’s version, which makes you a lot faster than the normal version.
And then they go. Teleports into position all over the city, two hundred people all of them up under a Communal Mind Blank that’ll keep them at least for the first hour of the fighting. The fortress wall gets Disintegrated in precisely the right location not to collapse it - they don’t want to collapse it - but to let them in.
Where the layout is not, in fact, as their spies saw it. Changed around in the last few days, which means this was anticipated, which means -
Sudden darkness, when it was previously broad daylight, dark enough that even those with darkvision can’t see their glowing swords in front of their faces.