Valley of the Brain Collectors as a sandbox had plenty of optional encounters. Here is one which my players missed. Converting monsters from 3.5e to 5e is more art than science and its time consuming. So wherever possible I don’t convert at all I replace. For example I didn’t convert Hyrsek Caio (assassin), Harab (assassin again), The Penitent Androids (gladiator) or Mad Paeytr (druid) I just used straight stat blocks from the MM. But Nemgedder Janz was unique enough to be converted.
NEMGEDDER JANZ 
Medium undead, neutral evil
Armour Class 19 (red scatterlight)
Hit Points 221 (20d8 +60)
Speed 30 ft.
STR 15 (+2) DEX 25 (+7) CON 16 (+3)
INT 10 (+0) WIS 13 (+1) CHA 15 (+2)
Skills Perception +3, Stealth +9
Damage Resistances necrotic; bludgeoning, piercing, and slashing from non-magical weapons that aren’t silvered
Damage Immunities poison
Condition Immunities exhaustion, poisoned
Senses darkvision 60ft., passive Perception 13
Languages the languages it knew in life
Challenge 11 (7,200 XP)
Stranger’s Luck (1/Turn). A pale stranger may roll an addition d20 on any attack; ability check or saving throw roll made and take the result he chooses. He may also use this as a reaction if not used in his previous turn to force an attacker to accept the same d20 result.
Sunlight Sensitivity. While in sunlight, the pale rider has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
ACTIONS
Multiattack. The pale stranger makes two sonic pistol attacks. It can use its Life Drain ability as a ability as an attack rider to add an addition affect.
Sonic Pistol. Ranged Weapon Attack: +9 to hit, range 150/600 ft., one target.
Hit: 13 (1d8 +9) thunder damage plus 12 (1d6 +9) necrotic damage and Life Drain.
Life Drain. The target must succeed on a DC 17 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
Stranger’s Shot (1/Turn). The pale stranger deals an extra 13 (4d6) necrotic damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the pale stranger that isn’t incapacitated and the pale stranger doesn’t have disadvantage on the attack roll.
DESCRIPTION
This nearly skeletal figure wields a pistol in each hand. Its garb is dusty and weathered, and smells of old decay.