Level 100

Level 100 in a skill requires that you first attain level 99 in that skill, then drink a so-called "[100 Potion]". You will be prompted to select a skill in which you have level 99, then it will be permanently boosted to level 100. This level is only needed for a handful of spells and creations, such as [Annihilate], so unless you desperately need to use one of these, it would be advisable to stick to level 99.