take upon oneself

昨日取り上げた "fealty" が使われた文に慣用句らしい "take upon himself" がありました。
In acknowledgment of fealty, he proceeded to take upon himself the guardianship of his master’s property.
"take upon onself" で辞書をみます。
・The American Heritage Idioms Dictionary: 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.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: take something (up)on oneself: to accept the entire burden of something on oneself. (Upon is formal and less commonly used than on.) You didn't need to take it all upon yourself. There are others here who can help, you know. Jan takes too much on herself.
責任を持って仕事を引き受ける感じですね。White Fangの場合は引き受けると言っても自主的に行っていますが。