Zhul'rhaan is an interesting encounter to say the least. It's an encounter intended to creep your players out, to draw them separately, and to fool them into thinking they know what's going on. This creature can perfectly mimic sounds it's heard, and will use that to its advantage. By design, it's not very strong. The creature will take advantage of confusion to use hit and run tactics on the humanoids it draws into its lair.
Zhul'rhaan rules by separating the party. It lures in the separate party members by mimicking voices its heard, making the individual members think they're running towards their friend, then it strikes. This creature isn't blatantly strong, it relies on weakening its opponents before finally confronting the weakened foe. It should perform a series of hit-and-run tactics to do so, and only confront the party when it has properly lowered their fighting spirit or it gets cornered. Once it identifies it can take an opponent down, it doesn't hesitate to ruthlessly assault.
Zhul'rhaan is a cold beast - very calculated, and makes its moves carefully. It won't just mindlessly utter phrases it has heard, it will use key ones to lure people away. Once it determines it can safely remove a foe, it will. Until then, it will toy with its prey. They say fear makes the meat more tender, after all...
"Hey! Over here, come help!" You hear your friend call out. You look in the direction it came from, and see a cave.
"Gera? Why did you go into the cave?" You call out, trudging towards it.
It's dark inside, but you spark a torch alive, venturing in further. The cave has a lot of twists and turns, but you follow the sound of your friend beckoning you in.
Eventually, the twists end, and you enter an open room. Venturing to the center, you stop being able to hear your friend. You start hearing a soft growling come from the way you came. You quickly jerk around, thrusting the torch towards the sound to ward off any whatever the source of the growl is. Perhaps you ventured into a wolf den?
But...there's nothing there. From behind you, you hear your friend call for your help again. You twist around quickly once more. You're met with the sight of a horrific purple and blue four legged beast with barbed tentacles sprouting from its back. It lets out that same soft growl. Then the growl stops and you hear something else emit from the beast.
"Hey! Over here, come help!"
Zhul'raan, as written, is created from a human - magic being cast upon them to twist and contort the body until this horrific abomination stands in the person's place. While the person exists in this state, they are fully conscious but unable to control their body. They CAN still exert some effort to have various effects, but that is the extent of their abilities now.
The right music helps set the tone amazingly. There are really two stages to this encounter - the hunting stage and the final showdown.
During the Hunting Stage, something unsettling and mysterious would really help convey the tone. The Gaunter O'Dimm theme from The Witcher 3 is an excellent track for this purpose. Alternatively, if you do normally play music, playing NO music, but having ambient sounds going does wonders to confuse and concern your players.
During the Final Showdown, something faster paced, hectic, but still unsettling would be an excellent go to. Something along the lines of the Taniks Reborn fight music would hit the spot.