take it upon oneself
Also, take on oneself. Undertake something, as in I took it upon myself to count the precise number of children in the audience, or She took it on herself to enter a convent. [Second half of 1400s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.