noun, noun or participle which takes 'suru'
arrival at one's post; taking up a new appointment
Formal term used when someone arrives to assume a new position, especially in official or organizational contexts.
新部長が来任した。
The new department head arrived at his post.
来任の挨拶を済ませる。
I will make the customary greetings upon taking up my new post.
着任 is more common and neutral for 'assuming a post'; 来任 emphasizes the arrival aspect and is more formal.
Compound of 来 (come) and 任 (duty, post). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.