A Dish Served Cold

This is the mission that is available, when choosing the "Revenge" option, after the cutscene where Pegorino asks you to do a deal with Dimitri Rascalov.

After getting a phone call from Roman who tells Niko that Dimitri is on the "Platypus", the ship in which Niko arrived to Liberty City in, Niko heads to the docks in Broker. Once there, Niko kills Dimitri's guards to get to the ships controls. Upon arriving at the controls, Niko opens up the cargo hold in which he finds Dimitri. Niko kills more of Dimitri's guards before killing him aswell.