Yesterday I was looking over the conjuration spells for Oblivion, and I realized that there isn't a spell to summon a dread zombie. Most undead and daedra you can summon, and you can summon progressivley superior forms of each undead/daedra as you level up. For example, skeletons have 4 main types, which can be summoned (Skeleton; novice level, skeleton guardian; journeyman level, skeleton hero; expert level, and skeleton champion; which is also expert level but costs more magicka and doesn't stay as long as a skeleton hero.) Atronachs have 3 types, each more powerful than the one before, however they use different elements (Flame atronach; journeyman, frost atronach; expert, storm atronach; master.) Then there are other miscellaneous daedric summon spells, such as scamp, clanfear, daedroth, and spider daedra. In addition to two dremora conjuration spells, being summon dremora (Summons a caitiff) and summon dremora lord (Summons a Markynaz). The gamemakers probably didn't put all of the dremora types into conjuration spells because the seven different types of dremora wouldn't match up evenly with the amount of conjuration skill levels (Novice, apprentice, journeyman, expert, and master). But I'm getting off topic. If you check the wiki (And in game expierience) you will know for a fact that you can summon every type of undead that isn't a custom enemy (Undead akaviri soldiers for example, are a custom type of undead enemy), so why can't you summon dread zombies? They're about as powerful as some of the other master level summons (Xivilai usually have less health than dread zombies) and the're not too strong, since gloom wraiths are much stonger. Hope you found this useful, and please do tell me if you find a summon dread zombie spell in the Elder Scrolls IV Oblivion.
-Korak Nar, The Necromancer