Any illusion spell you cast with a duration of “concentration” lasts a number of additional rounds equal to 1/2 your wizard level after you stop maintaining concentration (minimum +1 round). At 20th level, you can make one illusion spell with a duration of “concentration” become permanent. You can have no more than one illusion made permanent in this way at one time. If you designate another illusion as permanent, the previous permanent illusion ends.
