Killing captains and warchiefs in Middle-earth: Shadow of Mordor earns you runes which you can put on your weapons to make them better.
Get Better and Epic Runes
The level of the rune depends on the power level of the caption or warchief you kill. The higher level the rune the better the rune is. So to get better runes you need to increase the power level of captains and warchiefs. You can do that by deliberately dying to them or helping them succeed at hunting challenges or riots. You can also put a death threat on them to increase their power level and increase your chance of getting an epic rune from them.