Intellect Devourers have the devour intellect ability. This means that if you fail a save your intelligence is reduced by a certain amount. If a wizard with a 20 intelligence, at level 10, he would have a DC of 17 and a spell attack bonus of +9, if he fails the save, and his intelligence is reduced by 10, would this also reduce his DC to 12 and his attack bonus to +4?
Devour Intellect only has an ability-score-reducing effect if the rolled “reduction“ equals or exceeds the targets intelligence score, in which case it drops to 0 INT. Otherwise, the INT score is unaffected.
The Intellect Devourers ability is worded as follows (emphasis mine):
Devour Intellect. The intellect devourer targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 12 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll3d6: If the total equals or exceeds the target's Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence.
As we can see, Devour Intellect can only either reduce your intelligence score to 0, or not at all.
For example, if you have 16 INT and the Intellect Devourer rolls 15 on his 3d6, you are not affected, aside from the psychic damage.
Yes... If it actually worked that way.
Attack bonuses and save DCs are derived from attribute modifiers, and attribute modifiers are derived from attribute values. If the attribute goes down, the reduction chains through to the attack bonuses and DCS. The same thing would happen to a Fighter's attack rolls, and a Battlemaster's ability DCs, if his Strength or Dexterity were reduced.
Devour Intellect is all or nothing. If the roll isn't high enough to reduce the character to Intelligence 0, it does nothing to the character's Intelligence score at all.