As much as you hate it, it looks like your only option is to try and talk your way out of this. It's not nearly as straightforward or direct as using your hooves to beat the message into their bodies, but it's your only choice.
Think about it. You're tied up, hoof to wing. You can't use your items, you can barely wiggle let alone fight! Clearly, the only solution is to talk them down. And the only way to talk to a crazy... is to speak crazy.
"Wait!" Much to your surprise, they do. Both cultists freeze then lower their daggers from stabbing height to a more casual stance.
"Yes?"
"Don't I get a chance to argue my case?" you blurt out.
White Cloak and Black Robe share a look. "What's there to argue? Death is—"
"—inevitable. Right, got that. So since you're gonna kill me anyway—" hopefully not "—does it matter if I take a few seconds to try and convince you otherwise?"
"You won't," White Cloak says coldly. But in a hairpin turn his tone brightens again. "But you have a point." He slides the dagger back into its hidden sheath. "Make your case. You have one minute."
Right. So you've bought yourself a little time. Now to formulate a convincing argument as to why their philosophy is wrong and they shouldn't kill you.
...
...
You've got nothing. The problem lies in that they're right, strictly speaking. Death is inevitable. Even the princesses will die someday, even if not just from old age like everypony else. They're not truly immortal, just really good at putting it off.
Suddenly, it's like a lightbulb goes off. That's a loophole. One you've already made use of. Now you just need to expand it. Good thing crazy ponies accept crazy logic.
"You know, one minute might not be enough. If I'm going to die anyway, does it really matter if happens immediately or five minutes from now?
"I... guess not." Black Robe sounds unsure of himself as he replies.
You press the advantage. "So if five minutes doesn't really mean anything, why not push it to ten?"
"So long as you still die at the end of it?"
"Of course."
"Alright then."
A wide grin starts to creep across your face. Who knew outsmarting ponies could be this fun? Is this what Twilight feels like all the time?
"And compared to how long forever is, is there really any difference between ten minutes and an hour?"
Black Robe hesitates before looking to his partner for guidance. After a moment of deep contemplation, he nods. "Forever is a long time."
Feeling confident by your string of successes, you decide to push your luck. "And hey, just putting this out there, maybe you could untie my ropes in the meantime?"
You realize your mistake as the both of them suddenly tense back up and reach for their daggers. "I'm not going to run away or anything!" you say quickly, "I mean, where would I go? Death is inevitable, right?"
"True enough." White Cloak steps closer and cuts your bonds. Sharp prickly needles dance across your hooves and wings and full bloodflow returns.
"Thanks." You swing your legs around so you're sitting on the edge of the altar.
"No problem. Need a drink?"
You shrug. "Sure." Reaching into the recesses of his cloak, he pulls out a water bottle and hands it to you. You give it a sniff before drinking. Doesn't smell like poison. The first sip doesn't taste like poison either. It's weirdly thick for water, but tastes fine. You gulp down half of it then pour the rest over your head, cleaning out your mane where the dust of the mansion had merged with the sweat formed by thrashing against the ropes into a gross tan goo.
"Thanks again," you say as you pass the empty bottle back. "You know, you guys are alright when you're not trying to kill me."
"We are still trying to kill you." Black Robe's voice is as calm as though he's talking about the weather. "Death is inevitable. Yours is just pushed back for—" he turns away for a moment, staring at nothing "—fifty-seven minutes and forty-three seconds."
"Cool, cool." Cryptic commentary aside, it's hard to take them seriously as threats anymore. "Actually, you know I think I have plans to meet up with some friends in an hour. Mind if we pick this up in a day or two?"
White Cloak raises an eyebrow. How he conveys that through a mask so clearly you have no idea. "You'll still die?"
"Of course. Death is in every table."
He nods at your great wisdom.
As you walk towards the door, your thoughts begin to become clearer. Most importantly, you remember that this is a haunted house and that you were never actually in danger. You're slightly ticked off at Starlight for misleading you about there not being any actors, but you can forgive her in turn for the element of realism the surprise added.
Reaching the door, you turn back to face your two new... friends? Cultbuddies? Thespianmates?
"You two are really good at this whole thing. You got any plans later?"
"I plan to return to the earth so that my body may become nourishment for a new generation of insects."
You nod. "Hm. Ambitious." You have to give him credit for not breaking character. You can always ask Starlight about them later.
"So uhhh... bye."
"Before you go," you turn back to find both of them right behind you instead of back where they had been on the platform. Did they teleport? "A gift, for one who understands our philosophy so fully."
White Cloak reaches into his pockets and pulls out a simple brass Alarm Clock. "A timepiece. That you may always remember exactly how much time is left and how fleeting it is. May it aid you when time seems meaningless."
Black Robe reaches into his own robe and someone pulls out a lit Lantern. The flame dances inside the glass housing despite the perfectly still air. "And the Sacred Flame. A powerful aid against those who refuse to accept that their time has expired."
You accept both with a word of thanks and place them into your bag. As ominous as the warning was, you're not sure how a clock is going to be useful. The flame though sounds very handy to have around in case any zombies or ghosts show up in future rooms.
You pry open the door and step out, sticking your head back in for one final goodbye. "This was kinda fun. I'll see you guys around. And uh... Remember Death."
"Remember Death," they intone as one.
You let the door slide closed.
The two cultists stand alone in their silent chamber, staring intently at where you just were. Slowly, the torches begin to flicker and dim. Shadows grow from every corner, gnawing and twisting. Square-edged stones seem to curve and bend in the strange light, a spiral growing from the gridwork.
Light continues to seep away, till naught is left but the utter darkness of the void. From it comes two voices, sourceless and indistinct.
"Time is but a doorway."
"Death is but a window."
"She'll be back."