The fully advanced Necronaut from the Monster Manual III. I'm not 100% certain what CR to use. It's certainly not a +1 CR to +1 HD ratio. If you look at the official ratio of HD to CR, you'll see that the CR is equal to 0.4375 HD. This would put the fully advanced necronaut at CR 42 flat even. On the other hand, this thing is far too weak to be CR 42... after all, as CRs go up, usually you expect the variety of abilities to go up as well!
Utilizing Upper Krust's CR document (version 5), we come up with a CR value of 32, which sounds more reasonable to me. That's what I'll go with. Note, version 6 (which I don't own) of the document suggests that when advancing creatures, add 0.3 CR for every Intellegent Undead HD added, which would bring us to CR 33, just one off the value calculating it full out with version 5 offers. Aparently, using version 6 to calculate it full out suggests that it should be CR 28 instead. So consider CR 28-33 the range for this thing. It's somewhere in there for sure.
Fully Advanced Necronaut CR 32
Genderless Necronaut Advanced +64 HD
Chaotic Evil Colossal Undead (Chaotic, Evil, Extraplanar)
, Senses ; ; Darkvision 60 ft.
AC 59, touch 1, flat-footed 59 (-1 dex, -8 size, +68 natural)
hp 1301 (96d12+672 HD); DR 15/lawful or magic
Immune Intellegent undead immunities; SR 43
Weakness Can be turned, Necromantic Effects
Spd 70 ft. (14 squares)
Melee Slammelee ( /19-20/ )
Base Atk +48;
Space 20 ft. ft.; Reach 20 ft.
Atk Options Awesome Blow, Great Cleave, Improved Bull Rush, Improved Overrun, Improved Sunder, Power Attack, Trample (DC 82)
Special Actions Assimilate Corpse
Abilities, , Con — , ,
SQ Bone Armor**, Unholy Toughness, Undead traits
Feats Alertness, Armor Skin x6, Awesome Blow, Blind-Fight, Cleave, Great Cleave, Greater Critical Multiplier (Slam)*, Epic Weapon Focus, Greater Weapon Focus (Slam), Improved Critical (Slam), Improved Critical Multiplier (Slam)*, Improved Bull Rush, Improved Initiative, Improved Spell Resistance x9, Improved Natural Weapon (slam), Improved Overrun, Improved Sunder, Power Attack, Weapon Focus (Slam)
Skills Knowledge (The Planes) +10, Survival +2 (+4 other planes)*New Epic feats from the Immortal's Handbook.
Assimilate Corpse (Su): As a move action, a necronaut can add any non-animated corpse within its reach to its body. Once the corpse is added, the necronaut begins assimilating it. On the following round, the necronaut heals an amount of hit points depending on the size of the corpse.
Size of Corpse
Hit Points Regained
This process of assimilation does not take an action once the necronaut has added the corpse to itself. Once the assimilation is complete, the corpse cannot be raised from the dead. Before a corpse is fully assimilated, it can be pulled free with a DC 24 Strength check, drawing an attack of opportunity as the ally enters the necronaut's square.
Necromantic Effects (Su): Spells that create undead can be used to harm the Necronaut for 1d4 damage per caster level, up to 10d4 for Animate Dead, 15d4 for Create Undead, and 20d4 for Create Greater Undead. Epic spells that create undead deal as many d4 in damage as the maximum HD creature that spell can create.
Trample (Ex): As a full-round action, a necronaut can move up to twice its speed and literally run over any opponents at least one size category smaller than itself. The creature merely has to move over the opponents in its path; any creature whose space is completely covered by the trampling creature’s space is subject to the trample attack. If a target’s space is larger than 5 feet, it is only considered trampled if the trampling creature moves over all the squares it occupies. If the trampling creature moves over only some of a target’s space, the target can make an attack of opportunity against the trampling creature at a -4 penalty. A trampling creature that accidentally ends its movement in an illegal space returns to the last legal position it occupied, or the closest legal position, if there’s a legal position that’s closer.
All trampled creatures are dealt full slam attack damage, except as follows:
Trampled opponents can attempt attacks of opportunity, but these take a -4 penalty. If they do not make attacks of opportunity, trampled opponents can attempt Reflex saves (DC 82) to take half damage.
For the most part, I followed the advancement rules flat out. I increased the size to colossal as per the advancement rules. I also added stat bonuses every 4 HD (+16 stat increases, 8 to str, 8 to cha).
One thing I didn't follow the core rules for advancement was for speed. Instead, I used the Immortal's Handbook's size guidelines to advance the speed. I matched the necronaut's normal speed of 50 ft to a medium quadruped's speed +10 feet, so for the colossal necronaut, I took the large quadruped's speed +10. Larger creatures simply have a larger gait.
**Additionally, I gave the necronaut the Bone Armor trait, which is a design spec from the Immortal's Handbook, stating that natural armor increases by one for every two HD added. The original necronaut should have had this feature, which would have given it 5 extra points of natural armor. The +68 natural armor listed above is +48 Bone Armor, +14 Colossal Size, +6 Armor Skin feats.
Here are the factors that went into determining the CR.
First, we calculate the ECL, with the following factors.
ECL 96 Intellegent Undead HD +43.2 Undead Type Traits (Intellegent) + 1.1 Colossal Size (w/Ability Score mods) + 5.6 Trample averages 21 damage by dice + 1.05 Ad hoc for Corpse Assimilation + 0.2 (feat equiv) ------ 51.15 Silver Rule 48.15
Then, we follow Krust's simple rule that CR = 2/3 ECL. Which in this case equals 32.