OK We have been playing around with the Undead Lord and truthfully find it seriously lacking. ESPECIALLY when it comes to the Corpse Companion.
Unless your party slays a lot of creatures with racial HD, you are going to run into a situation where your corpse companion will never exceed 2 HD as a zombie. Even taking on templates as our GM suggested either allowed us to get an out of control situation or still a lacking creature.
SO I have attacked the idea from a standpoint similar to that of a druids animal companion. Many of this is from a variety of source books for 3.0 or earlier but here we go.
Corpse Companion (Su)
With a ritual requiring 8 hours, an undead lord can animate a single medium undead companion with Hit Dice equal to there cleric level. This is a unique type of undead that may resemble a corpse in various states of decay chosen at the time of the ritual.
The base companion stats are as follows at 1st level.
Corpse Companion
NE Medium undead
Init +0; Senses darkvision 60 ft.; Perception +0
DEFENSE
AC 12, touch 10, flat-footed 12 (+2 natural)
HD: 1d8
Fort 1/2 HD , Ref 1/2 HD, Will 1/2 HD +2
Immune undead traits
OFFENSE
Spd 30 ft.
Melee see below
STATISTICS
Str 10, Dex 10, Con —, Int * Wis 10, Cha 10
*Intelligence is = to the Clerics WIS Bonus.
Base Atk Bonus: equal to 3/4 total Hit Dice
FEATS
Based on chosen form below
SKILLS
Skill points equal to 4 + Int modifier (minimum 1) per Hit Die.
The following are class skills for undead: Climb, Disguise, Fly, Intimidate, Knowledge (arcana), Knowledge (religion), Perception, Sense Motive, Spellcraft, and Stealth.
Choose a Form
Skeletal: DR 5/bludgeoning;
Feats Improved Initiative
Melee Attack 2 claw (1d4)
Abilities +5 STR +4 DEX
Withered: DR 5/slashing;
Feats Toughness ;
Melee Attack slam (1d6)
Abilities: +7 STR
As the undead lord advances in levels the Corpse Companion gains 1 HD as well. As it gains power it also gains improved positive channel resistance preventing it from being turned, am increased Natural armor as well as being imbued with different Special Abilities, and an Aura.
Instead of adding a table Here is the basics.
Corpse Companion gains another feat at every off level. (Its 1st level feat is based on its chosen form)
It gains an improved Ability score at 2, 6, 10, 14 and 18 HD
Its Natural AC starts at +2 and increases by +1 at 4,7,10,13,16 and 19 HD (max +8)
It has +2 Channel Resistance at 1 HD and gains an additional +2 at its 9th and 20th HD.
It gains a an Aura at 7HD and the improved Aura at 19HD
It gains a special ability at 2, 5, 10, 13 and 18 HD.