I am about level 86 in Illusion. When I cast invisibility, everyone can still see me. If I am being attacked and cast it, they still chase me and hit me.

When I cast frenzy at a group of enemies, they all continue to run at me instead of fighting each other.

I have the quiet casting perk, yet when I cast a spell while sneaking, the eyeball opens up and people look for me.

What is going on?

Well frenzy only works up to about level 5 so you need to update you spell to Rage I think it's called. The casting perk only works on casting spell quietly, it won't make you unseen just unheard when casting spells.

You still need muffle or sneak for invisibility to be effective. Its a rather worthless spell in my opinion.

I think you need to be naked in order to get the invisibility work.

try at home 1st.

This isn't Mystery Men or Eric Cartmen in South Park.

If I need to sneak for invisibility to work, then why use it? Sneak works great on its own if you level it and perk it.

I kept reading about how awesome illusion is, but 30 hours in I think it is kind os worthless. I wasted almost a whole tree worth of perks.

Not at all, Illusion is my favorite magic school. If you're invisible I can still hear you, so just cast muffle and with sneak you'll never be found. Illusion has some great spells you just need to use them in the right way.

Of which quiet casting and invisibility are the least important imho, never used them much. Rage and calm are extremely powerful.

Illusion is very powerful. While invisible and either having cast muffle or wearing a combination of the DB boots/Muffled Movement perk, it is possible to sprint past enemies without being detected. I rarely use fear, however frenzy and calm (and their higher level equivalents) allow for very efficient killing.

I'm currently playing a stealth character who only wields a dagger. It's almost impossible for me to kill anything without a sneak attack, so illusion spells are mandatory for clearing groups of enemies or if I get surprised by a sabercat, because I will die in 1-2 hits.

I've never had this problem with Illusion. I've used Fury while sneaking and havn't had trouble. If there is no one else nearby sometimes will look around and find you before attacking others, but if they see someone else first you're fine.

When I was working on my "Master Criminal" achievement, I used Invisibility while running away from the enraged citizens and guards. It didn't fail me at all, as they quickly lost track of me.

For Fury/Frenzy, you should ideally move out of their sight after you've cast it. A frenzied target will attack anything, including yourself. So, don't distract him and his friends from taking care of each other.

lol at worthless comment.

Anyways, just muffle yourself, and invisibility combined with that will make you undetectable until you attack.

The thing that I think people don't realize about Invisibility is that when you cast it in combat, the enemy will still search for you around where you disappeared. If they run into you, or if they hear you nearby, they start swinging.

No being invisible only reflects the light around you hence you're invisible to the eye. If you fart I'm gonna hear you, so muffle your sound. It makes perfect logical sense.

Yup, cast, jump to the side, swing for __x (fill in sneak attack multiplier) damage.

Fear is also good in some situations. The usual downside is that fleeing enemies will alert their friends but sometimes that can give you a better chance to frenzy a group. Also, sometimes fleeing enemies will get stuck in a corner and you can attack them freely.

There are so many uses if you have enough magicka. That's been the biggest drawback for me; some of the spells take a lot of magicka especially if you have to dual cast them for high level mobs and I'm always on the edge. I think the next time I'll focus on fortify Illusion instead of Destruction.

That's when I use it, being a thief I'm not the strongest in a fight so making one run away helps the odds. Calm is good for wild animals.

In addition to what others have said, you need Kindred Mage, Rage and Dual Cast perks to make your frenzy spells work on high level opponents. For Calm, you need Kindred Mage, Hypnotic Gaze and Dual casting.

If you can't get a Frenzy or Calm spell to work, try dual casting it. That doubles the level the spell will affect.

As far as quiet casting, that one has always worked for me. Are you within sight of enemies when you cast? The light from casting a spell (or even holding your hands out with a spell equipped) can be seen and quiet casting won't help with that.

The calm spell is also great for picking pockets. A calmed enemy won't attack you even if he catches you with your hand in his pocket.

I find Illusion very efficient.