That depends on you. And, to some degree, on them.
There are some things you just can't say IC, but many people at least appreciate the effort of an IC message, especially if they are a faction with an emphasis on roleplaying. You can always put an OOC addition on the end if there's more you weren't able to phrase IC.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."