I came to Oblivion after Skyrim so at first I was amazed at the creative possibilities of making your own custom spells. Skyrim magic feels so bland now in comparison, I don't understand why they removed this feature from the game. But anyway in this topic I wanted to share some of my favourite custom spells. I'm probably late to the party and everyone will know these already but it took me a long time and a spellbook full of failed spells to finally figure out to create effective spells.

Chain Heal - Fortify Magicka 100 pts for 3 secs, Drain Magicka 3 pts for 3 secs, Restore Health 30 pts for 1 sec, Restore Fatigue 30 pts for 1 sec

Very useful in tight situations since healing spells are often the ones that require the most spamming. This one means that you can quickly block cast your way back to full Health/Fatigue and jump back into melee combat. The Fortify Magicka/Drain Magicka effects give you a temporary reservoir of Magicka to keep repeatedly casting the spell from, i.e. if you repeat the spell within the 2 sec window you won't completely exhaust your Magicka. Ideally by the time you need more health/fatigue your Magicka will have replenished.

Fire Barrage - Fire Damage 20 pts on target for 1 sec, Weakness to Fire 100% on target for 3 secs, Weakness to Magic 100% on target for 3 secs

Ice Barrage - Frost Damage 20 pts on target for 1 sec, Weakness to Frost 100% on target for 3 secs, Weakness to Magic 100% on target for 3 secs

Lightning Barrage - Shock Damage 20 pts on target for 1 sec, Weakness to Shock 100% on target for 3 secs, Weakness to Magic 100% on target for 3 secs

These are always hotkeyed for me. 3 casts of the Shock Spell are enough to take out a Daedroth on normal difficulty, and very magicka efficient. They make Destruction actually usable between skill level 50 and 100. I only recently discovered spell stacking courtesy of another thread here and have to say they have completely opened up Destruction magic for me.

Marked for Death - Drain Speed 100 pts on target for 1 sec, Weakness to Magic 100% for 8 secs

This is a sneaky spell that you can cast while undetected. The Drain Speed temporarily disables the enemy, preventing them from noticing the weakness to Magicka curse you've placed on them, and thereby detecting your position. You can add in other weaknesses, e.g. to poison or another element, but I like to then sneak up to the enemy and use...

Soul Crush - Soul Trap on Touch for 1 sec, Drain Health 100 pts on Touch for 2 secs, Weakness to Magic 100% for 2 secs

A very cheap but deadly spell that can take out pretty much any enemy in 1-2 hits, especially if you've hit them with 'Marked for Death' first.

Horseradish - Fortify Speed 15 pts on Touch for 120 secs, Waterwalking on Touch for 120 secs

Very useful on your horse if you're planning on exploring the Bravil / Leyawiin area. The Niben just becomes one giant highway!

Please share your own spells. I haven't been playing this game for very long so I'm sure I've only just touched the tip of the iceberg.

Some of mine may only be useful at lower levels, but here are some of my favorites:

Turn and Burn - Turn Undead + Fire Damage (same duration as the Turn Undead) + Light (for a few seconds longer). Can be made on Target, on Touch, or one of each. Sends undead running away from you, on fire, and illuminated. The Light duration is set longer than the damage so that you'll notice them coming back at you if they survived the spell.

This is very useful at lower levels because of the lower magicka cost of the component spells and the fact that the enemy isn't fighting you while taking the DOT fire damage, allowing your magicka to recharge a bit before they come back for another dose.

Finishing Touch - Soul Trap 1s, Drain Health 100 pts 1 sec. Similar to your Soul Crush spell. This takes advantage of how low cost Drain Health spells are. At lower levels this is an instant kill of pretty much everything. It is also handy to use after you have worn them down with other means if they have more than 100 HPs. If the target still has more than 100 HP, then this spell essentially does nothing so you do have to adjust tactics as you level up. Improved versions (once you can add Weakness to Magic) include a 3-second WtM effect, so that the second application does 200 points of damage if they recovered from the first one.

Killbolt - Same spell as Finishing Touch, only it's ranged. Makes an exceptional snipe weapon from stealth, assuming it 1-shots the foe. Improved Killbolt adds a 3-sec Weakness to Magic and you just fire two in rapid succession. I also create "Notrap" versions of both so I don't accidentally fill up a big soul gem with a small soul.

Sow Dissent - Frenzy Level 25 for 10s in 25 ft, Invisibility on Self for 10s. Can be ranged or on touch. Handy for when you get mobbed. Everyone fights everyone else, you can scurry out of the middle of it. Durations can be adjusted for your level and magicka costs.