Broken, but still Standing...

Confronting Mandilissimar in his tower is a challenge, but isn't meant to be debilitating. He has been tormented by the psychic whispers of the disembodied demilich, driven to madness. The madness would fall under the category of indefinite madness as outlined on p. 259 of the DMG. This creates a flaw that lasts until they are cured, dictating a lot of their actions. Mandilissimar has become consumed with his research, in particular locating the artifact he is searching for. He has become obsessed with locating and recovering it despite anything that might interfere.
Unfortunately, he is unaware of the extent that he has already gone to in his search until the undead start waking under the power of the lantern. In that moment, he becomes aware that his obsession has led him along a dark path. He is able to shake the control of the demilich in his revulsion, but the effort causes a shock to his system that brings on a long-term madness that lasts 10 hours. This break from reality sends him rushing back to his tower, overwhelmed at his own depravity, and causes a temporary amnesia to engulf him.
This amnesia, coupled with his revulsion at himself, and a healthy paranoia, leaves him in a state of mind to aggressively defend himself and his tower against assault (real or imagined). If the party arrives, he will regard them as hostile and retreat to his upper floor to rain spells upon the perceived threat. He can pose a legitimate threat while in this state, but if he can be subdued magically or physically, the amnesia will pass and he will just assume a defeated state. He will still be focused on recovering the artifact he seeks, and will begin to develop a new paranoia toward the party if he understands they have recovered anything of value from the manor.
Within the library of the mage, secured in his private study beneath the first floor, there is a collection of spell scrolls and an old map showing a rough trail leading up into the great mountain southwest of Aron's Bowl, toward the volcano holding the dragon's lair. There is little marked on it that is decipherable, but Mandilissimar can fill in the details if his madness is cured. Otherwise, the party will have to assign their own meaning to its worth.
The map shows a route Mandilissimar followed to reach the dragon's lair on one of his quests years ago. Needing access to the dragon lair for spell components, he negotiated his way through the rancorous fens that spill down the side of the volcano. In full mind, he can relate some of the terrors of that journey, plagued with elemental creatures and a tribe of slaad living in the mire. Without his full wits, he can only rant about the horror and treacherous earth of the route.