Here is the rest of the battle as for what a mezzoloth is I will have to find out for you.
Fate of the Nameless One
The Nameless Chosen stood before the Silversgate, driving the Whipmaster Cvor before him, and he laughed at the mezzoloth's challenge. As Nameless turned to bid his comrades-at-arms good will and luck in battle, the razor-studded whip of Cvor wrapped about his throut, and the two fell into the gate, struggling. Flying to his aid, Elminster the Crafty entered the Silversgate and shut the gate behind him to isolate Cvor's March. The Nameless Chosen's full intent in his mad attack was to open up the Army's lines and isolate sectors of it to destroy more easily. Alone but undaunted, the fire-engorged form of the Nameless One towered over the battle. Back to back with the elder Chosen Elminster, he fought hundreds of mezzoloths within the rocky Silverymoon Pass, and the glow of his silvery form drew attention from Silverymoon. Still, while his successes led to victory for the Allies of Cormanthyr, the hubris of the Nameless One led to his undoing.
Trusting that his flaming form would be proof against any attacks, he was caught unawares by Colonel Cvor when it used a captured elf artifact (the Harness of the Giant-Slayer Alayris) to grow to a giant's size and seize the flaming mage by hand and foot. More swiftly than the reactions of either Chosen, Cvor brutally pulled the Nameless Chosen nearly in half by sheer strenght! The explosion of magical power and fire utterly consumed Cvor and over 200 nearby mezzoloths, but not the one wielded it. As the fireball subsided, the Nameless One lay dying, the silver fire desperately draining away out of his laid-open torso. While Elminster later reported that Mystra herself preserved his life as he lay wounded, at lest three sources (two half-elf histories and one elf song) suggest that another power such as Sehanine saved him in return for his aiding her People. Still, the High Mage of Silverymoon Ecamane Truesilver and his supporting wizard forces protected the body of the fallen Chosen from the remaining mezzoloths. Together, while the Nameless One was sent magically to Silverymoon for healing, Elminster and the elderly and frail Eamane destroyed the gate to Myth Drannor, detonating it from within and at its exit. The explosions slew the remnants of the Fourth Regiment surronding it at Myth Drannor as well as clearing Silverymoon Pass of its remaining brethren.
As a result of this battle, Myth Drannor's forces won the day, but unknowingly paid a high price. Elminster was lost among the planes for a time, due to his task of destroying the gate from within, while Ecamane Truesilver died due to the strain of breaking the gate from its exit point at Silverymoon Pass. The Nameless Chosen lay like one dead for more than the next year, though Mystra spoke to those who tended him, calling him "he whom magic, duty, and honor defines." His elf nurse, who heard this in her own tongue, gave him the name Akhelbhen; upon his awakening, the Nameless Chosen, once called Arun's Son is introduced him to the new High Mage Aganthol as Khelben Arunsun. His body healed but forever scarred across his chest and back where he was nearly torn in twain, he banishes his vanity and keeps the wedge of silver-white hair and beard to remaind him of the silver flames dancing within it and how it does not make him invincible. It also signifies his debts, as it reminds him of Silverymoon and what he owes its people as well.
There ends the making of Khelben "Blackstaff" Arunsun the Mage of Waterdeep.
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