I know what you're thinking, folks. Don't worry, he's killable.

I was looking through a few of the boss ideas - Greavers, Devilite CEOs... and I was thinking, maybe we could have an undead boss (not Vana, I mean) as an alternative endgame boss? So it came to me - I could make one of the most hated enemies - Phantoms - even more hated. Hurr hurr.
Haven't exactly come up with a name yet, but I was considering Baron Blackheart.

Storyline
Even some Phantoms aren't satisfied with the status quo. One particular phantom sought great power even in the afterlife - and went all out to achieve it no matter what the cost.
This Phantom amassed immense amounts of dark energy from claiming the lives of unwary Knights and absorbing its weaker companions, eventually rising above all others. Known only as Baron Blackheart - his former name, he has become a legend of the Clockworks, a cautionary tale to young, brazen Knights on wandering graveyards alone. The foolish few who sought to challenge him barely made it back alive.

Appearance
http://img254.imageshack.us/img254/3713/phantomboss.jpg
Essentially a massive Phantom (about twice the size) with armor that looks similar to a Fallen Set, only with a black cape. Instead of the usual purple glow, Baron Blackheart has a black glow. His weapon of choice is a long spear with what looks similar to a massive Sealed Sword on the end, with Baron Blackheart's characteristic black glow. Baron Blackheart is resistant to all other damage types but Normal and Elemental, and resistant to all statuses but Slow and Fire - only in the floating skull form.

Phase One - The 'Phantom Phase'
This phase is only available before reaching the actual boss level. Think of him as the worst possible phantom - ever. He will chase you to the end of the level, and takes about 5x more hits than a regular Phantom to temporarily disable. To make matters worse, you have a window of 15 to 20 seconds before he chases you again.
1. A charge attack firing 5 Gran Faust charge attack blades in succession, each with a chance to curse.
2. Three swings dealing up to 5 bars of damage each. Generally easy to avoid/block, as the attack speed is comparable to that of a Troika. But be warned that the swings have a huge range, about 1.8 times that of a GF.
3. Summoning 4 to 6 zombies upon 'death'. The zombies drop hearts upon death.
4. The most annoying skill - teleporting away followed by attacks 1 or 2 on another player. This is highly unpredictable, not to mention very, very painful.

Phase Two
This is where the actual boss fight begins. By now, the boss fight will take place in a massive, barren graveyard-like map with no gravestones. The boss comes charging in with a massive skeletal horse, and will be riding the horse through this phase.
1. Body Charge Attack. Fairly simple to dodge.
2. Firing seeker skulls that dish up to 4 bars of damage.
3. Summoning zombies randomly.

Phase Three
The horse crumbles to dust, and the boss steps off. The boss is extremely agile in movement (think of his movement speed and agility like a Striker in Lockdown). After sufficient damage, the boss teleports and engages another target - the one that dealt the most damage to it. The boss only follows one player around at any time.
1. The boss' weapon swings at the speed of a Sealed Sword.
2. The boss teleports more frequently. Each teleport releases a nova that curses players.
3. The boss now acts as a Grim Totem.

Phase Four
The boss and his weapon will emanate a bright red glow, and in this phase he become highly dangerous with new skills:
1. The boss' weapon swings at the speed of a Spur.
2. His Dash moves 150% faster than a Striker in Lockdown
3. The boss teleports much more frequently, and will dash out the way of attacks.
4. The boss can put up a shadow shield to block attacks for a few seconds - and when destroyed, it stuns all players in range.
5. Instead of Zombies, Phantoms will start spawning. Unlike normal Phantoms, however, these Phantoms also gain the ability to teleport, followed by unleashing their charge attack instantly.

Phase Five
The boss turns permanently into a blue floating skull, marking the start of the final, and deadliest phase:
1. The boss can still use a charge attack that fires up to 8 swords in succession.
2. While in floating skull form, the boss pulsates occasionally with a nova that curses Knights hit by it.
3. The boss teleports on every alternate hit.
4. The boss' general movement pattern is in the form of circling the followed player at high speeds.
5. The boss' "Grim Totem" effect instantly revives downed Phantoms.

Do post some suggestions or additions to this idea if you like, I'm still thinking of ways to improve on it.

EDIT: I took Khamsin's feedback, and made only the Second Phase seem familiar to Vanaduke. The rest of the battle puts you up against an extremely agile boss.