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Death Knights have a long and storied existence in D&D, but this is the first time I hear of Hell Knights.
Hell Knights are absolutely not Nazgul, no way, no how. They’re narzugons, which is a completely different thing. Jokes aside, their origins are quite tragic.
The first hell knights used to be servants of He Who Was, the god Asmodeus rebelled against. Asmodeus tricked them into betraying their former master, and their betrayal damned them to serve Asmodeus forever. From that point forward, whenever someone got tricked into betraying their deity, they would become new hell knights upon death.
I think this is the first time I see an explicit mention of people going to Hell and becoming devils in 4e. This used to be a 3e thing, so I guess Hell Knights come from that edition. It could also be that angels and other immortals become hell knights when they get tricked into betraying their deities, if you want to play it that way.
Though hell knights are powerful, becoming one is definitely a form of punishment. A Hell Knight’s existence is an eternity of hatred. They hate themselves for their betrayal, they hate their new masters, they hate their immortality, and they hate free mortals. Though many of them serve Asmodeus directly, every potentate of Hell has at least a few knights at their disposal. The most famous group of them is the Order of the Nightmare, so named for their preferred mounts. Other groups have other mounts, like the mammoth-riding knights of Cania, or the ash riders of Avernus with their wyverns.
Hell Knights usually only associate with members of their own order, but will work together with other devils when commanded to do so.
Hell Knights are Medium Immortal Humanoids (devils), and Level 15 Soldiers with 146 HP. They have Darkvision, are immune to fear, and have Resist 20 Fire. Their ground speed is 6, but they will almost always be mounted on something faster. Their gear is the full “Classic Knight” setup of plate armor, heavy shield, a lance, and a mace.
Their Hell’s Rider trait allows their mount to use the knight’s Athletics, Acrobatics and Stealth scores if they would be better than the mount’s own. The mace does bonus fire damage and knocks prone on a hit, and the lance has better base tamage and crits on a 19-20 when used on a charge. Both count as basic attacks.
As a minor action they can use Eyes of the Narzugon, which causes a creature within 10 squares to be marked and slowed for a turn. This is an effect, so it auto-hits.
Hell Knights pair really well with Nightmares! As I mentioned in the Nightmare article, enemy riders can still take actions independently of their mounts, so the knights can charge with their lances and maces at the same time their nightmare mounts set the map on fire. Since they have native fire resistance, they can ignore the burning terrain even when unhorsed.
A simple but effective hell knight encounter would consist of three knights and three nightmares, using classic medieval cavalry tactics (charge early, charge often). This group’s lack of ranged attacks would be compensated by the nightmare’s high teleport speed.