A few I have sent in the past while invading:
"Give me all your valuable loot and I may let you live, cur"
"Send your phantoms away and I will make your death quick"
or, just to antagonize, " Your souls and humanity shall be mine, weakling"
...of course when you lose you look like a jerk, so don't send straight hate mail, but most people recognize that you are just joking and send something funny back. It's a good way to get a friendly rivalry going. If somebody was obviously ganking or used some cheap tactic to kill me I never send a message.
A few messages I have sent Sun-broing:
Upon greeting-
"Lead on, fearless master!" followed by
"My ______(sword, spear, whatever I am using) is at your command, sir!"
Upon dying before boss defeat-
"Master, I am not worthy!"
"Forgive my weakness, I beseech thee!"
And upon victory-
"For all my days I shall remember fondly our victorious adventure, sir!"
"We have defeated great monsters together, brother, always know that I am your friend!"
Anybody else ever do this?