Here you go.
Points to note:
This build is optimised to use a +4 Intelligence Helm (readily available from Thayan Enclave in Sshamath Markets for about ~400,000gp; it should be the first major item you should look at saving up for).
Feel free to raise/lower your DEX/STR/CON/CHA/WIS as you like to suit your RP. You can either aim for Odd stats (eg Str 9/ Dex 15/Con 13) and aim for +3 stat items (common enough) OR Abyssal Might spell, or go even stats and use +4 Bulls/Cats/Bears.
Starting INT must be 20.
Make your background feat whatever you want (I chose bully because that's what drows do
)
Skills: Your free choice, besides the usuals of Concentration & Spellcraft. Pale Master will unlock the Hide & MS skills, which are good RP tools in the UD even if you are not a HIPSer, as are some of the custom Lore skills BGTSCC has. As this build does not have Able Learner, cross classing skills will cost 2 points each. I would recommend 10 ranks in Tumble (+1 AC) and 11 ranks in UMD if you can spare it (not essential). Your Spellcraft Skill synergy should boost your UMD by another 2 points to end at 13 and able to use wands/items from any class/alignment/race.
Level 1: Spellcasting Prodigy, Necromancy Specialist School; Opposed School: Your pick (I recommend Divination but Evocation is also a possibility due to -4 CL for that school, but you will lose out on having situational Bigbys & IGMS. You won't need Magic Missile as you should be using Negative Energy Ray instead to heal your undead summons. I'll leave that call to you.)
Level 3: True Believer (SA qualifying feat)
Level 5: Extend Spell (SA qualifying feat. You may choose Empower here, but as a DC caster you should be focusing on single shot spells that kill/fear/paralyse your foes so your summons can mop them up. Extend spell will help counter -4 CL for conj/trans, especially if you take SA at 6th)
Level 6: Toughness (BM qualifying feat)
At level 6 you must decide whether to advance your spellcasting, or advance your survivability.
To advance your spellcasting, start taking Shadow Adept at level 6 (1 Level of Blood Magus at 7 for Blood Component), and then continue SA until 9th level. At that stage you are FORCED by the 3b20 rule to take 3 levels of PM & 2 more levels of Blood Magus to have your split as Wiz 5/ SA 9/ PM 3/ BM 3 at level 20. Note that by taking this route your character will have a good spell book (Wizard 19 spellbook at 20th level), good DCs in Necro/Enchant/Illusion but -4 CL in Evo/Trans/Conj throughout. This is significant because when you take your first level of SA at 6th your Conj/Trans/Evo spells will have an effective Caster level of 2, which means hardly any duration at all! Hour/Level spells will still be fine, though (like Improved Mage Armor). Use scrolls of Spiderskin at this stage because they have 9 CL, +4 AC and will last 150 minutes per scroll (or until dispelled). Take Abyssal Might as a 4th level spell to help compensate for this fact because it lasts 10 min/CL (10x more than Bulls/Bears/Cats). Because you're UD, no one will care how often you use this spell.
The other drawback is not having those superb PM summons to help you throughout the levelling process, but having (Greater) Spell Focus Necromancy will give your undead summon spells a needed HD boost (BGTSCC custom change: Spell Focus Necro/Conj will advance the HD of creatures summoned from those schools).
Being in UD will be a distinct advantage because you can switch to using Animate/Summon Undead to tank for you, and the bonus CL from SA will boost their duration significantly. You can also use spells like Conjure Shadows/Summon Shadow pack as they switch to the Necromancy school for SAs (not their usual Conjuration school) for summoning oomph in specific battles. Utilise Negative Energy Ray (1st) /Negative Energy Burst (3rd) instead of Magic Missile/Fireball/Flame Arrow to heal your undead summons and also deal damage to mobs as necessary.
The Spellcasting option will find its defensive peak much later in your build (level 28) when you finally get Deathless Mastery and immunity to critical hits, but your general spellcasting power will be significant throughout, meaning you can take advantage of your boosted Necro/Ench/Illus DCs to sleep/confuse/fear/hold/dominate or just plain kill your foes while your summons work on the mop-up.
The survivability method pretty much just swaps the progression of Shadow Adept & PM. This route will have lower CLs throughout the levelling process (Wizard 16 spellbook at level 20 so up to 8th level spells, but you will get full spellbook into epics of course), but your AC will be higher, you will have strong PM summons to tank for you throughout the levelling process and, if you find yourself struggling a little, you can take an earlier 2nd level in Blood Magus for Scarification (simply swap an early 2nd level of BM for a later level of PM), which is the ability to write scrolls on your character's skin for use whenever you need them. You will receive Critical Immunity at level 21 if you take this route (it is the route listed on my link).
Because your first SA levels can come at levels 18-20 via this method, you can swap True Believer to a later feat and take Practiced Spellcaster earlier. You also will not suffer from -4 CL to evo/trans/conj for the early levelling process, meaning by the time that penalty comes into play your caster level will already be significant enough that it won't really matter (esp. when combined with Extend Spell to buff your summons with bulls/cats/bears).
The survivability route will peak in power in mid-late epics as your SA levels significantly boost your CL and your DCs, making you a true engine of destruction.
Good luck!
I might have missed some finer points; some of the more expert builders will pick up on those and post below if necessary.