When in Ruler mode and you obtain Lu Meng, he asks you to leave on a journey to 
train and learn. You have no say in the matter, but for a few months, his troop 
count falls to 1 (as in just himself). You are free to use him, but this is not 
wise, considering the effects of troop numbers in battles, which is very 
important compared to Officer mode. When he returns, he obtains a second 
ability, "Flames" (though it may be random), and his troop count returns to 
normal.