Kannik
Legend
Luckily this is a text-based game so I went back to see what exactly the prophecy said. It seems I quoted the CoS book exactly-
Center: "The final destination. Your enemy is a creature of darkness, a being with powers beyond mortality. This card will lead you to him."
Maybe even use the Abbot from the monastery. He is a fallen angel and could play into that.
I've not read (nor played) CoS yet, but there could be a fun and interesting combination of several of the suggestions presented thus far: When the PCs arrive at the room foretold by the deck, they meet the Abbot (or someone prior they considered an ally). Whether the Abbot is free and walking about, acting all nice, or is bound and thus "needs" to be rescued is up to you... the fun part is whether the players/PCs remember the prophecy and thus recognize that the Abbot is the evil. Have the Abbot try to get them to leave.... maybe it's part of his plot that if nothing is disrupted in here (an altar, the heart, coffin...) their plans will come to fruition. Maybe he tries to ambush them. Letting the scene play out, with the Abbot acting just weird enough so that the players begin to suspect something... until they remember the cards and that this was where they were supposed to meet the enemy. That moment of dawning comprehension. Then all heck breaks loose.

However it goes, here's to it turning out wickedly cool for you and the group!
Kannik