Most expensive spell to cast, in terms of Magicka?

[Adam08]

What is the most expensive spell to cast, in terms of magicka, and is it possible to create a character that can cast it without the use of mods/buffs?

[Loki[D.d.G]]

Are you referring to vanilla spells or those you can create? If it is the latter, then there is no definitive answer to the question. Just load up multiple effects on a single spell and see the magicka cost rise

[Adam08]

Naw I'm referring to spells that are available on the 360 version, so the not-as-fun spells that console owners are limited to. It is only possible to have so much magicka which is why I want to know how I should make a mage character.

[Fljotsdale]

Naw I'm referring to spells that are available on the 360 version, so the not-as-fun spells that console owners are limited to. It is only possible to have so much magicka which is why I want to know how I should make a mage character.

Never bothered finding out which specific vanilla spells cost most magica, but they are all Master spells, and by the time you are capable of casting Master spells, any decent spellcaster (High Elf, or Breton, with Apprentice sign), should have sufficient magica to cast 'em. Using certain classes of spell (Mysticism, Summon) increases points for INT on level-up, plus you can enchant items to give you extra INT, etc.

[Belthan]

minor spoilers

Finger of the Mountain is a spell you get as a quest reward (spoiler:

Spoiler

it's an optional side mission that opens up during the Mages Guild quest line after you seek the recommendation from the Chorrol guild

), and the magicka requirement can be so high that it's impossible to cast, depending on what level you were when you did the quest and what level of spell you used to solve the puzzle. I've seen the number be in the thousands when I did the quest at high level and used an Expert level spell, so it's best to do it while your Destruction skill is low and use a Novice level spell to solve the puzzle (the magicka requirements for any given spell decrease as you skill in that school increases, so if your Destruction skill is already high when you get the spell, it won't have much room to come down in magicka cost).

Also, once you get access to an altar of spellmaking you can create your own spells even on the 360. You get access by (spoiler:

Spoiler

getting far enough in the Mage Guild quest line

) or with the Frostcrag Spire DLC. And as Loki pointed out, when you create spells it's pretty easy to make a spell so powerful that you can't cast it.

[Tricky]

I think paralyze might have the highest magica use per second. Not sure though.

[ParanoidAndroid]

I think enemies explode is the most expensive (other than custom spells), see here:
Oblivion:Spells - UESPWiki

842 base cost:
Fire Damage 5pts in 10ft for 5sec on Target
Fire Damage 70pts in 20ft for 1sec on Target

[MageRage]

I do not about any "game-wise" but my FireFostShock was BAD A**.

-FireFostShock-
Fire-70 for 3 sec
Frost-70 for 3 sec
Shock-70 for 3 sec
Dude it was epic!!!
It was cool because it showed up like "shock".

[Fljotsdale]

Yeah, we've all done that sort of thing. But we figured the initial question was about puchasable spells, rather spells created 'in character'.

[Master David]

It is only possible to have so much magicka which is why I want to know how I should make a mage character.

Permanent CE glitch.
It is legal because beth knows it and didn't fix it (when beth was still actively patching Oblivion).

[MageRage]

I think enemies explode is the most expensive (other than custom spells), see here:
Oblivion:Spells - UESPWiki

842 base cost:
Fire Damage 5pts in 10ft for 5sec on Target
Fire Damage 70pts in 20ft for 1sec on Target

Then I agree.
Mine is still epic!