I remember Dragonfable, is it still running? I remember getting bored trying to improve my potions.
Anyway, I'll take a crack at designing him when I get home from work.
Cool, lets design Sepulchure, first off my assumption is that you are the GM and are willing to bend a few rules to make this happen. Thats important because you are not going to find anything Cookie Cutter in Pathfinder to make Sepulchure. Its gonna take some home-made craft work to get him working.
Step one is clearly Level 20 Anti Paladin, that right away gives you DOOOOM powers with Touch of Corruption along with the Antipaladin spell list, access to heavy armor, and the ability to channel negative energy which is extremely important for controlling undead. Basically, it makes him good at melee and a capable spell caster when it comes to Undead.
Step 2, Mythic this b@~@# up. Sepulchure is clearly M10, the biggest bad. Etc, We're going for a Hierophant/Marshal duel path. Important feats include Duel Path, Mythic Command Undead, Mythic Spell Lore (selecting Animate Dead + 9 other spells), Channel Smite, and Mythic Undead Master.
For our Marshel and Hierophant powers we want Advance, Beast's Fury, Mounted Marshel, Mythic Companion, Clarion Call, Contingent Channel Energy, Channel Shockwave, Shape Channel, Shatter Resiliance, Master of Mercy, Divine Source, Binding Ritual
These make our Mount Stronger (one of the locations where we will be cheating things a little) and enhance our ability to channel energy.
Step 3: Templates, we've got a Crazy strong Antipaladin here but we need to make him ever more powerful. That is where templates come in. Specifically, I can think of 3 useful ones. However, we are going to need to pick and choose abilities from these a little bit to make them fit Sepulchure.
We are taking Death Knight as is, except we are ignoring the mount, its pretty damn strong and fits Sepulchure pretty well. However, we are picking and choosing from the Graveknight template. In addition to once again ignoring the mount, we are swapping out their Elemental Energy affinity and ability to revive after death. Sepulchure wouldn't rejuvenate if killed, someone else would take his place by holding the sword and wearing the armor. Instead, we're turning the damage straight to negative energy instead of fire, acid, cold, or electricity. Lastly, we slap on the Unholy template just really because we can. Also, if you wanted to keep him human instead of an Undead by ignoring the whole type change from the other two templates, then this is the best way to get him to be healed by negative energy
When we look at the HD we have access to at this point, assuming everything stacks, we have 80 from Animate Dead and 20 from Command Undead. Then we have another 40 from Death Knight and 100 from Grave Knight, which gives us 240 HD of undead. Less, really, because now we get to the mount.
His personal Mount can be a Ravener Dragon, I'd go Red but the color is up to you. Go with something powerful, but not too advanced along the age chart for his personal mount. After all, you want to save the Great Wyrm Red Ravener Dragon to be his flying castle.
Feel free to slap on Mythic Skeleton Template to these dragons for the extra kick.
Anyway, thats the legwork I'd go through. Slap on PC wealth, give him an artifact or two like a +6 Spellstoring Conductive Unholy Greatsword, and show him to center stage as Bad Guy Mc Evilton for the campaign.
Just make sure the end result is comparable to the other CR enemies you plan to hit them with. Take an enemy or two of the appropriate CR and compare AC, saves, and the like to make sure you haven't accidentally cooked up Sepulchure the wrong way and made him unbeatable.
Remember, hes gonna be base CR 20 as a level 20 character with PC wealth. Then you add on Mythic, his templates, any draconic mount/allies that participate in the encounter, and an extra CR per artifact that he's gotten ahold of.
Anyway, try not to make him more powerful than Baba Yaga or Cthulhu since he is going to be encountered with minions. Budget the XP from Sepulchure and his undead dragons together when determining the final CR