Prelude: Into the siege lines

Ailech had been under siege for only four days but already the situation was becoming dire. The unnatural forces of the Undead had appeared as if from nowhere in enormous numbers and had been pounding the walls of the city with infernal catapults incessantly since the siege opened. Refugees were being taken out of the city by elven ships and those merchantmen able to sail but a strange,  pall had fallen over the river which had destroyed the boom within hour, burnt through the River gate and spreading along the shores and into the harbours damaging ships, docks and wharves and soon he remaining ships would have to leave or be left sitting in the silt of Ailech's harbours.
In an effort to buy time before the walls were breached Lord Erkenfried and the Silver Elf Admiral Tirieth Starbow hatched a desperate scheme. Small bands of warriors drawn from all the defenders would go out into siege lines under cover of nightfall and destroy the screaming skull catapults.
On the west wall, between the towers of the Watergeat and the Westgeat, three brave souls volunteered with their personal retinues; the self-styled Dwarf mercenary Kronin the Barbarian; Silver Elf woodsmaster Siriel Greencloak and the militant bishop Aisans Guire of the Border Kingdoms. Lowered from the walls the heroes headed towards the river  hoping to outflank the Undead artillery in the darkness. They had been warned of undead "patrols" but had no way to know how many or where these patrols would be.
Misfortune befell the heroic warriors quickly though. As they moved cautiously over some difficult ground by the river bank they met a large patrol of undead infantry and archers. The elves at the forefront of the group were taken by surprise and with uncharacteristic emotion let loose a swift hail of arrows and made an impetuous charge into the middle of the patrol. Some of the elves fell to a return volley from the skeletons but when melee was met the sorcerous power of undeath was little match for elven steel. The clash and clamour though drew more skeletons to the fray but as they moved towards the elves in an effort to overwhelm them with numbers they were distracted by movement on the hill above. Suddenly the clash had turned into a melee. Noticing the bishop's retinue a number of the skeleton infantry turned up the hill toward the shield line, but Kronin's dwarf band in their turn were not about to let tall-folk have all the fun and led by his cousins, the berserks Throrin and Thali, the dwarfs tore into the approaching undead tide. The fighting was swift and deadly with all the skeletons falling to axe, spear and sword with little injury in return. The bishop's wizard to cast a shielding spell but failed causing himself to be dazzled with his own enchantment.
After some healing and regathering of their forces the heroes resumed their flanking manoeuvre, this time being more lucky than before as a number of suspected patrols and ambushes turned out to be false alarms. on the final approaches to the catapult emplacements, the dwarfs, the lightly armed but effective borderers and a few of the elves were surprised as a large force of undead cavalry  came crashing across the empty fields of the abandoned village to the east of the undead lines. On the hill, scene of carnage only minutes ago, the retinue of the bishop himself and the remaining elven archers formed a line and prepared to sell their lives dearly to provide the main strike force by the catapults time to finish their task. In the confusion, as the catapults fired blind into the oncoming warbands, a stray screaming skull fell amongst the gatehering forces and felled three of the borderers while Greencloak was seemingly killed outright. Afterwards in the retreat back to the city though, no trace of his fallen form could be found...
Throrin the berserk leapt into the trenches of the undead lines, as the undead horsemen crashed up the hill towards the stalwart defenders and the fighting now became truly desperate. On the hill, the melee circled and wheeled back and forth. Sometimes the defenders were on top, other times their straits seemed dire indeed. Several of the retinue fell and the elven healer Moontouch was killed but ultimately the line held and several of the defenders aided by the remaining elven archers and infantry dispatched the last of the vile horsemen. Throrin meanwhile had been fighting a similarly back and forth fight against the wight leading the undead forces until eventually, as his dwarven comrades and the remaining borderers broke into the trenches, he struck the wight down with a truly mighty axe blow. The trench fighting was quickly over after the wight fell as dwarfs and humans frenziedly hacked, slashed and smashed skeletal bodies and infernal constructs to pieces.
The triumphant, though battered forces made their way back to the rendezvous but before the signal could be given and ropes dropped from the walls an almighty inward rush of air blasted over the walls into the city and an enormous crash heralded an explosion in Valjevo Castle, the inner keep and the head-quarters of the city defence. Rubble and ash fell to the ground all across the city and it was obvious from the growing, fey, green glow in the eastern quarter of the city that something truly evil had occurred.