I cannot hope even to come close to the "Tome of Necromancy" by Frank and K for advice--it should be mandatory reading for any necromancer--, but here're some starting points for manufactured undead.
Creatures Available Using Animate Dead
The 4th-level Sor/Wiz spell animate dead [necro] (PH 198-9) lacks the versatility of the 6th-level Sor/Wiz spell create undead [necro] (PH 215) and the 8th-level Sor/Wiz spell create greater undead [necro] (PH 215), but Wizards of the Coast didn't ignore it completely.1 In addition to the skeleton (MM 225-7) and skeletal dragon (Dr 192-3) and the zombie (MM 265-7) and zombie dragon (Dr 197-8), the following undead can be created via the spell animate dead by meeting additional requirements and providing the additional components beyond onyx worth 50 gp per HD.
- The bloodhulk (MM4 22-4) is available as a fighter (10 HD), giant (14 HD), and crusher (20 HD). Manufacturing a bloodhulk requires corpses of the appropriate size (Medium for a fighter, Large for a giant, and Huge for a crusher) and the "blood from three recently slain creatures the same size as the potential bloodhulk." Note: Bloodhulks count as double their Hit Dice when determining both if a creature can use the spell animate dead to manufacture one and for the total amount of undead HD a creature can control.
- The bone naga (MM2 35-6), according to "Birth of the Dead" (Dragon #336 38), is created by a ritual known to dark nagas (MM 191-2) via the spell animate dead with additional material components including "the ocular fluids of a divine caster and a sentient reptile" (which may or may not be in the necromancer's spell component pouch right now). However, the ritual is "[o]nly taught to dark nagas [and] contains... somatic components that humanoids cannot emulate."
- The dread (LE 169) has as an additional material components "a zendalure gemstone worth at least 1,000 gp."
- The necrosis carnex (MM4 104-5) requires a minimum caster level of 11 and requires as part of its manufacture "three corpses from Medium creatures and cold-hammered iron bands worth 200 gp" that become part of the creature.
The Necrosis Carnex Meets Your Requirements
While it would be beautiful to create bone nagas, unless the DM's willing to handwave the fluff (and, really, the DM shouldn't--bone nagas are ridiculous and the fluff required to learn and perform the ritual is pretty cool), the necrosis carnex is the winner. Although it only has an Intelligence score of 3 and can't speak (it only "understands [its] creator's orders"), it can nonetheless lead by example and even gain class levels.
Have the DM determine if necrosis carnexes "make few attempts to communicate" (MM4 105) because they're--I don't know--just jerks or something, or if it's because they physically can't communicate. If the former, necrosis carnexes don't need the feat Surrogate Spellcasting (SS 39-40) to become casters, but, if the latter, they're out of luck without modifying all their spells with the feats Silent Spell (PH 100) and Still Spell (PH 101). Even lacking class levels, the smallest army of manufactured undead should contain at least one necrosis carnex anyway; its supernatural ability necrotic touch makes it an undead army's medic (This can be superseded by quantities of the hazard black sand (Sa 20), which can be created temporarily via the 3rd-level Clr spell black sand [necro] (Sa 111)).
"Can Hit Dice Be Added to Undead?"
A creator can't add Hit Dice manufactured undead as part of their creation (q.v. here). Manufactured undead have a set amount of Hit Dice and must gain Hit Dice via either acquiring class levels, a special ability, or fluff text (e.g. undead who exist long enough sometimes acquire the template evolved undead (LM 99-100)). However, a variety of means exist to upgrade manufactured undead as part of their creation.
Methods of Upgrading Undead During Their Creation
Too many base and prestige classes to list grant bonuses to manufactured undead, but of particular interest--and available to any creature with who takes even a single level of wizard--is the necromancer specialist wizard variant class feature enhance undead, an extraordinary ability, which says that
Any time a necromancer using this variant creates an undead creature (such as with animate dead, create undead, or create greater undead), all undead creatures created gain a +4 enhancement bonus to Strength and Dexterity, and two additional hit points per Hit Die. This ability does not affect the number or Hit Dice of animated creatures that the
necromancer can create or control. A necromancer using this variant does not gain additional spells per day for being a specialist wizard.
Emphasis mine. As the creature is still a necromancer after taking the level of wizard no matter what other classes the creature possesses, undead manufactured via clerical spells get this same bonus.
- The general feats Bolster Resistance (LM 25), Corpsecrafter (LM 25), Deadly Chill (LM 25-6), Destruction Retribution (LM 26), Hardened Flesh (LM 27), and Nimble Bones (LM 29) all improve the feats' possessor's ability to animate or create undead.
- The necromatic feats--all of which increase an undead creature's effective Hit Dice when computing the number of undead a creature can control and many of which have additional requirements to apply their effects to an undead--appear in "Among the Dead: Necromancer Archetypes and Abilities" (Dragon #312 29-38) and include the feats Bolster Power, Death Curse, Graveborn Expert, Graveborn Warrior, Heightened Agility, Heightened Strength, Uncanny Speed, and Vicious Claws.
- The 2nd-level Cleric spell desecrate [evoc] (PH 218) grants undead created or summoned within the area +1 or +2 hp per Hit Die and significant bonuses while the undead remain within the spell's area. A similar nonstacking +2-hp-per-Hit-Die is available via the deadwalker's ring (CM 126) (4,000; 0 lbs.). Similar nonstacking bonuses are available via the magical terrain ebony ice (Fr 15), the supernatural danger dead calm (Sto 13), the 3rd-level Clr spell defile ice and snow [necro] (Fr 92), the 6th-level Sor/Wiz spell blackwater taint [necro] (Sto 114), and, undoubtedly, other means.
- The 9th-level Sor/Wiz spell plague of undead [necro] (SpC 158) is like the spell animate dead except with a vastly reduced material component cost (just 100 gp total--seriously) and undead created via the spell have maximum hp per Hit Die. Note: Read that again. That's awesome.
Barring shenanigans, one character can't possess all the feats that improve manufactured undead. There are just too many (even to detail, which is why I only sourced them). Further, too many such feats grant inconsequential bonuses that become consequential only all at once in a big pile. Instead of permanently taking these feats, if possible, before manufacturing undead, acquire them via the power psychic reformation [telepathy] (XPH 127) and, when finished manufacturing undead, use psychic reformation a second time to return to the original build. Sure, it's dodgy, but the game is cruel to make folks think that a +2 bonus to manufactured undead's natural armor via the feat Hardened Flesh is worth a 1/7 or 1/8 a character's total feats for his entire career.
Below is additional information that may help the character create a more formidable undead army.
The Spellstitched Template
As soon as a creature who wants to master undead is able, he should apply the template spellstitched (CAr 161-2) to an undead he controls--preferably to an undead he can keep in a safe place and somehow grant an obscene Wisdom score. If the undead's Wisdom score can be sufficiently increased while the template spellstitched determines what spell-like abilities the spellstitched creature can employ, just one spellstitched undead can make (un)life as a necromancer much more interesting, saving him mountains of gold and bottles of XP.
A Sample Spellstitched Undead's Selection of Spell-like Abilities
This assumes a Wisdom 19 or higher spellstitched undead. Spell selection is independent of what spells are available to the spellstitched undead's creator, and there're no restrictions on the spell lists from which the creator can pick spells. For additional craziness, caster level equals the spellstitched undead's HD, which can be... impressive.
Spell-like Abilities: 1/day--animate dead [necro] (PH 198-9), animate dread warrior [necro] (Una 48), animate infectious zombie [necro] (City of Stormreach 144), animate undead legion [necro] (Dragon #309 49), command undead2 [necro] (PH 211), revive undead [necro] (SpC 175-6), undead lieutenant [necro] (SpC 226); 2/day--benign transposition [conj] (SpC 27), mage armor [conj] (PH 249); 3/day--create magic tattoo3 [conj] (PH 55).
"I Need More Undead!"
Is there such thing as too many undead minions? Of course not. The holy site sepulcher of Wee Jas (CC 156-7), the armor and shield special ability undead controlling (DMG 219) (49,000 gp; 0 lbs.), and the magic items night caller (LM 79) (7,000 gp; 0 lbs.) (and whether this supersedes Azun-gund (SC 32) (3,000 gp; 0 lbs.) is the DM's call), rod of undead mastery (MIC 175-6) (10,000 gp; 3 lbs.), porcelain mask (OA 141) (27,000 gp; 2 lbs.), mace of unlife (AE 112) (138,312 gp; 8 lbs.), rod of restless death (ELH 139-40) (625,000 gp; 5 lbs.), and a host of other items allow a creature to control more undead than normally allowed.
The feat Planar Touchstone (PlH 41-2) while linked to the touchstone site Catalogues of Enlightenment (PlH 166-7) can give the character the granted power of 1 of the Domains deathbound (SpC 272-3), deathless (ECS 105), or necromancer (ECS 107), any of which will improve the creature's ability manufacture or control undead.
- Area G. Library of the Temple of Zoretha (ElE 57) contains a scroll of lesser animate dead, which is--seriously--a spell in Pathfinder. It's otherwise unmentioned in Wizards of the Coast products. So, y'know, at least the spell animate dead wasn't as ignored as that spell.
- Requires the undead to be capable of speech; alternately, break economies (albeit slowly) with the 2nd-level Sor/Wiz spell continual flame [evoc] (PH 213).
- Requires the undead have at least 1 rank in the Craft (calligraphy), (drawing), (painting), or other craft skill, and, ideally, be able to succeed on a DC 20 skill check with that skill; alternately, just go with the ol' standby, the 2nd-level Clr spell desecrate [evoc] (PH 218).