Does this trick work:

[Os1ris09]

Ok so I am working on my Ultimate Magus build and I came across something that I don't know will work or not.

Illumian with Krau Sigil
Beguiler 1/Wizard 3/Spell-thief 1/UM 10/Archmage 5

feats:
1-able learner
flaw- fell weaken
flaw- extend spell
3- split ray
6- master spell-thief
9- empower spell
12- quicken spell
15- chain spell
18- practiced spellcater (beguiler)

With the Krau sigil and the appropiate level modifications here is what I have for a CL assuming practiced spellcaster and Krau work together.

Wizard: 3 wizard + 2 Krau + 5 Archmage + 10 UM = 20
Beguiler: 1 beguiler +2 Krau + 4 practiced Spellcaster + 7 UM = 14
Spell-thief: 1 s.t. + 2 Krau = 3

total CL is 37 (34 assuming that spell-thief doesnt grant a CL).

[AvatarZero]

I'm pretty sure that Master Spellthief sets your caster level for all your arcane casting classes equal to (in your case) your character level. You get no benefit from the Krau sigil or Practiced Spellcaster, you will always have the option of increasing your Wizard casting via Ultimate Magus levels, and at character level 20 your caster level for all three classes will be 20.

[Biffoniacus_Furiou]

First of all, you should get Versatile Spellcaster from Races of the Dragon. It will let you spend two Beguiler spell slots of the same level to cast any spell you know of one level higher. That means you can go Beguiler 1/ Wizard 1 and as long as you've made the spellcraft check to understand a 2nd level spell, even if you haven't added it to your spellbook, you can now cast 2nd level spells via Beguiler spell slots. This allows you to go Beguiler 1/ Wizard 1/ Spellthief 1/ Master Specialist 2/ UM etc.

Second, you don't need Krau to add 10/10 UM to Wizard, Master Spellthief will do that for you. In fact, once you get Master Spellthief you no longer gain any benefit at all from Krau, as I'll describe later. See PHB page 58-59, the steps to leveling up a character. Specifically, at Step 7 you gain your feat, and at Step 8 and 9 you gain spellcasting advancements and class features. That means upon reaching level 6, you first take Master Spellthief thus equalizing all your caster levels, and then gain UM's spellcasting advancement which can be added to whichever class you wish.

"Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells." This is spellcaster levels, which means the level you use to determine your spells/day. Practiced Spellcaster and Krau do not affect this. Unless you were to multiclass into a class which did not grant arcane spellcasting of any kind, you will gain no benefit from Krau or Practiced Spellcaster upon gaining Master Spellthief. Don't forget about UM's Arcane Spell Power class feature, which can be added on top of everything else. Your caster levels at level 20 will be as follows:

Beguiler (X=8): ((1 class) + (7 UM) = X) + Y + Z + (4 ASP) = 31
Wizard (Y=18): ((3 class) + (10 UM) + (5 Archmage) = Y) + X + Z + (4 ASP) = 31
Spellthief (Z=1) ((1 class) = Z) + X + Y + (4 ASP) = 31
Caster Level: X + Y + Z + (4 ASP) = 8 + 18 + 1 + 4 = 31
Get a Ring of Arcane Might and Orange Prism Ioun Stone to add another +2 to your caster levels.

[Os1ris09]

Ok so would you recommend changing race to something else.

Also I like versatile spellcaster but how can you cast the 2nd level spell even though you made the check? To my understanding I can't cast it because I don't know any 2nd level spells even though I have the ability due to the feat.

Unless you are refering to the quite literal know and "ability to cast 2nd level arcane spells" then that would work technically but isn't that a cheese interpretation?

"Your spellthief and arcane spellcaster levels also stack when determining your caster level for all arcane spells." This is spellcaster levels, which means the level you use to determine your spells/day. Practiced Spellcaster and Krau do not affect this. Unless you were to multiclass into a class which did not grant arcane spellcasting of any kind, you will gain no benefit from Krau or Practiced Spellcaster upon gaining Master Spellthief. Don't forget about UM's Arcane Spell Power class feature, which can be added on top of everything else. Your caster levels at level 20 will be as follows:

Beguiler (X=8): ((1 class) + (7 UM) = X) + Y + Z + (4 ASP) = 31
Wizard (Y=18): ((3 class) + (10 UM) + (5 Archmage) = Y) + X + Z + (4 ASP) = 31
Spellthief (Z=1) ((1 class) = Z) + X + Y + (4 ASP) = 31
Caster Level: X + Y + Z + (4 ASP) = 8 + 18 + 1 + 4 = 31
Get a Ring of Arcane Might and Orange Prism Ioun Stone to add another +2 to your caster levels.

Also here you refer to spells/day, refer to bold area. Do you mean that I get additional spells beyond an 8th level beguiler and 1st level spellthief? That my effective spellcaster level is 27?

[Fouredged Sword]

That reading is up for debate. A conservative reading is that you can spend two spell slots of a lower level to cast a spell of a higher level, only working for spontanious casters, becuse wizards have "prepared spells" while spontanious casters have "slots" and not allowing you to cast spells from your spellbook, becuse you don't "know" those spells and it would be silly for a single feat to allow you to mix two spell lists like that.

It's still great. A level one sorrcerer can pull out a true striked fell drain acid splash to auto kill anything with one HD or less.