How do you summon a skeleton archer on oblivion and can you get the spell from any of the DLCs to?

In "The Elder Scrolls IV: Oblivion," you can summon a skeleton archer using the spell called "Summon Skeleton." This spell is available through the Conjuration skill and can be found in the spell tomes or purchased from certain mages. However, if you have the "Shivering Isles" DLC, you can also find a stronger version called "Summon Skeleton Guardian," which can be used to summon a more powerful undead archer.