Lord Randyll Tarly is a Lord and bannerman to Mace Tyrell of Highgarden. He carries the sword Heartbane which is obtainable if you manage to capture him and then get the option to steal his sword from him. This will effect your honor and your current relationship with him in a negative way.
Randyll Tarly has with him a large amount of Horn Hill knights. Making his army far more cavalry thick them most other Reach Lords.