During the battle with Trigon's minions, Beast Boy assumes the form of the Were-Creature he had in The Beast Within (2004).
Raven sacrifices herself to spare her teammates; She did the exact same thing in "The New Teen Titans" (vol. 1) #5 when she agreed to serve at her father's side if he would spare earth.
Cyborg proposes a game of "stankball" - a sport he and Beast Boy played in the episode Spellbound (2004).
Trigon's appearance at the end of this episode differs from his appearance in Nevermore (2003). But - it should be noted - the "Trigon" in "Nevermore" was a manifestation in Raven's mind. In reality, Trigon was redesigned because the creative team wanted to make him look scarier and cooler.
The storyline for "The End" was inspired by the comic book tale, "The Terror of Trigon" which originally ran in "The New Teen Titans" (vol.2) #1-5 where Raven had to fight Trigon's dark influence over her. This dark side grew as she exposed emotions. Eventually, Trigon came beckoning to Raven and she could no longer resist. Trigon's presence was unleashed! During Trigon's resurgence, Azarath was destroyed and Raven fell under her father's control, taking on her father's appearance as well (red skin and four eyes).