How can Illusion Magic be effective against True Sight?

Question

I am starting a new campaign and decided to play a wizard this time around. I was looking through the different schools available and reading guides (Treantmonk's among others) and saw illusion as one of the most liked ones. However, my DM and one of the other players have tried to dissuade me from this choice as a lot of higher level creatures (our last campaign went from 1 - 20) have true sight thereby making the illusions worthless.

My question is: How can an illusion wizard still be effective against creatures with true sight for a campaign that will extend to level 20?

Answer 1

There's an assumption in the question that I want to challenge here... I'm interpreting your question as "Can an illusionist be effective against high-level opponents with truesight?"

In D&D5E, all wizards select a subclass — most of them are school "specializations", but others exist. Taking a particular school does not bar the character from casting the full range of wizard spells¹, but give a few special abilities related to the type of magic they're best at.

True enough, your character's favored brand of magic would be less effective against creatures with truesight. However, by the time a wizard reaches that level, there should be plenty of other spells in the character's arsenal. As long as you don't neglect other schools, your character should be fine.

¹_{In previous editions, specialist wizards lost out on entire schools of magic to make them better in their selected school.}

Answer 2

Illusion magic is not only comprised of illusions. There are many mind affecting spells in the illusion school which are not affected by True Sight. To answer your question: you would be less effective against creatures with true sight compared to other types of wizards; however, you would not be useless.