Commander is a title which was unlocked by getting a score of 200-299 in a game of Mobilising Armies. It could be set to always appear and replace the current players' title when they enterer the Command centre. This could be toggled off by talking to any commanding officer in the tower.
As of 3 December 2018, it can be purchased for 100 thalers at Stanley Limelight's store.
This title changes according to the language of the server on which RuneScape is played, as shown below:
||Kommandant / Kommandantin [Name]
||Commandant / Commandante [Name]