The Revenant knight is a type of Revenant found in the Forinthry Dungeon. They are the undead version of Saradomin knights, however they have yet to receive their graphical update. It has the second highest combat level of all revenants (the strongest being the members only revenant dragon), and is the strongest monster in free-to-play (excluding Dungeoneering bosses).
Historically their combat level represented the maximum player combat level of 126 in free-to-play worlds. They have very high defence for their combat level. Their max hit with melee, ranged and magic attacks is very high, and will frequently hit over 1000.
Before the release of Summoning and the revenant dragon, the revenant knight was the most powerful revenant in the game. Then, after the release of the Evolution of Combat, the revenant dragon's combat level was lowered to 93, and the revenant knight regained its title again. However, they lost this title when the dragons were subsquently buffed again.
The Forinthry Dungeon is a PVP-enabled area, meaning it is entirely possible for other players to attack you whilst fighting the revenants in the dungeon. Do not bring equipment that you are not willing to lose.