Protection from Death Knight quest

When the PCs enter the city, the city guards talk to them.
"Excuse, are you adventurers?"
when the PCs say yes:
"Would you please come along?"
The city guards won't give any more information, except that the leader of the city wants to talk to them.

When they arrive at the leader, he starts a monologue:
"Greetings, I'm Tharaxus, the temporary leader of this city.
We have a problem, the last weeks several city guards have been killed.
We know who is doing this but we cannot stop him.

His name is Heskan, a death knight.
He has a grudge against leaders of this city and kills city guards until he gets to me. My predecessor has been kidnapped a couple of months ago by Heskan.

Heskan attacks only in the open, so I'd like to hire you for protection.
There is a reward of 2000 gp for killing Heskan."

Examining the quest

Tharaxus leaves out a lot of details as not to scare the PCs.
An Insight DC 22 reveals this, and if pressed, he'll tell more.
He also doesn't mention his predecessor, who is probably still alive.

History DC 16:
The city was once part of great empire.
The last king of that empire was betrayed by his lieutenant Heskan.
Heskan successfully killed the the king but was killed by the king's
loyal followers when he wanted to usurp the throne.

He didn't stay dead for long as someone wanted him still around.
Heskan, once a Dragonborn Paladin of Bahamut, was transformed in a
death knight.

Religion DC 20:
He spends most of his time in the Shadowfell torturing the last
leader of the city. If a new leader emerges he goes to the city
and kidnaps the new leader to torture him.
He only kills a leader if he has kidnapped a new leader.
This cycle has been going on ever since he became a death knight.

Streetwise DC 16:
He rides a black horse on fire, which the PCs identify as a Nightmare.
People have seen the Nightmare and Heskan appear and disappear,
riding though a blackish portal.
PCs who succeed on a Religion DC 18 know that the portal leads to the Shadowfell.

Stats

Heskan level 13 Soldier
Medium Undead
Initiative +11                     Senses Perception +16; Darkvision
HP 200; Bloodied 100
AC 30, Fortitude 27, Reflex 21, Will 22
Speed 5
Action Points 1
Saving Throws +2
Resist Necrotic 10, Vulnerable Radiant 5
Immune Disease; Poison

Standard Actions

Soulsword,        +19 vs. AC,    1d8 + 17 necrotic damage
Unholy Strike (At-will), +19 vs. AC,    2d8 + 21 necrotic damage
Sword Slash (At-Will; Range 10),
                        +19 vs. AC,   
                        3d8 + 16 necrotic damage, the target is knocked prone

Flame strike (Recharge 5 6 Bloodied; Close Burst 2),
                        +16 vs. Reflex,   
                        5d8 + 12 fire and necrotic damage,
                        undead creatures within burst deal
                        an extra 10 fire damage with attacks until end of next turn

Minor Actions

Dragon Breath (Encounter; Minor; Close Blast 3),
                        +20 vs. Reflex,   
                        2d6 + 7 fire and necrotic damage

Divine Challenge (At-will; minor; Range 5), the target is marked until
                        the death knight uses this power against another target.
                        If the target makes an attack excluding death knight
                        it takes -2 to attack and 14 necrotic damage.

Alignment Chaotic Evil Languages Common, Draconic
Intimidate +20, History +16, Diplomacy +18

Protecting the leader

The leader walks outside with his city guards, with the PCs nearby.
The city guards and leader are Human Guards (see monster manual).
The leader has double hit points compared to Human Guards.

Shortly after entering the streets, Heskan appears riding his Nightmare mount.
He gains fire resistance this way.

Encounter:

• Heskan
• Nightmare (see monster manual)
• Sword Wraith (see monster manual)

Heskan's background:
Shortly after adventuring in the Shadowfell, he gained companions:

• Nightmare mount, which allowed him to travel quickly from the natural world to the Shadowfell

• Sword Wraith, which he wears as a cape

Heskan won't say much, he'll charge straight for the leader.
Being mounted on the Nightmare gives him a speed 10.
If he notices the PCs he'll let the Sword wraith kill the city guards to create more Sword Wraith (Spawn ability).
The PCs recognize the Nightmare and may recognize and Sword wraith (Religion DC 19)

An important point to make is whether the PCs keep the Nightmare alive
so they can tame it for just one trip.
Taming the Nightmare means defeating it in combat, keeping it alive and make an Intimidate DC 18 to make it choose between service and death.
That way they can save the previous leader from the Shadowfell.
An alternative is to use the Shadow Passage ritual.

If Heskan is killed while Sword Wraiths are still alive, they will try to go into nearby houses and start killing commoners (or leader and city guards first if they are still alive).
If Heskan is killed while the Nightmare is alive it will try to escape and eventually go to the Shadowfell.

Also, who made Heskan a death knight and wants no leadership in the city?
This fact may lead to an antagonist.