Yeah, most of the disciple of *insert demon name here* and Thrall thingy are VERY cool and flavorful villain classes but require a decient investment into some negation skills, one of which is cross class for dread necros.(Persuade) So, with so few skill points as a dread necro, I want to put what few precious skill points I get into the important stuff- Spellcraft, Lore and Concentration. So yeah, although usefull, I would rather spend my few skill points on those needed attributes then waste them just to get into a PrC, making my character much weaker in the processes. The only way I could see myself taking any of those PrCs as a dread necro would be if I had a race that had some kind of large int boost, like Ithlids. However, the Ithlid race is VERY cheap and I want to stay away from it if possible.(Even though the idea of being a mini-Cthulu is very appealing.)
The shadow adapt sounds somewhat interesting, it adds some much needed defense to the DNecro, but there are better options for that(which I will name below.) As for the litch, it's pure useless for a DNecro, why? You need to be a lv.21 caster to get levels beyond 4 in it, and the dread necro stops leveling at 20. I suppose you could take one level in sorcerer or something, but I am not sure if that would qualify you for the litch levels above 4 or not. Also, I do not yet know what kind of Litch abilites the DNecro gets at Lv.20 in the game, but if the DNecro becomes a full litch, complete with abilities for free at 20, then the Litch class is even more useless for a DNecro.
The best thing I can think to do to advance a Dread Necro past lv.20 would be levels in Sorcerer. The Sorcerer bases it's spellcasting of the same stat the dread necro dose, but has a MUCH wider choice of spells, meaning you have the chance to pick up spells that can compensaet for what your limited DNecro list lacks. This means defenses, perhaps some better AOE/Crowd control and buffs for your undead. Of course, another interesting route to go would be Dread Necro/Favored Soul for acsses to the cleric spell list rather then the Sorc/Wizard one. Both lists are good, but the cleric list is VERY tempting since it has spells similar in function to the sorc/wizard list as well as inflict spells which can heal your undead army.(Imagine Mass Inflict Critical Wounds with your undead hord.....sounds VERY nice.) But then again, many of the spells on the cleric/favoured soul spell list are also on the dread necro list, while the sorc/wizard list has more spells that dread necros can't get normally.
Also, there are two other downsides to the favored soul. The first is the fact that although favored soul casting is based on the same stat as dread necro casting, the DC for favored soul spells are based on wisdom, which is usually the "dumb stat" of choice for arcane casters like the dread necro. However,if a favored soul can get high enough charisma and wisdom do cast spells, there is no reason a Dread Necro/Favored soul could not do the same. The other downside is more of a minor annoyance then anything else.....those blasted angel wings the favored souls get. I mean, nothing says evil overlord like a set of giant white angel wings sticking out of your back. I suppose if you had sephiroth hair(which is possible if you use one of the elvin heads(Some of which can look VERY sephiroth-ish)....but the whole undead skin that comes with being a litch just ruins the "one winged angel" look. If only there was a way to replace them with those bone wings Balors have, then we would be talking.