King Crimson is the stand of il Doppio con un Diavolo. The stand consists of 2 stands that play off each other very well, King Crimson and Epitaph.
King Crimson's main ability (and technically only ability) is to erase or skip time. The specifics on how he does it will be ignored as in a world of stands, vampires, buff aztec gods and aliens, it wouldn't make sense even if we do get it right. King Crimson will erase up to 10 seconds of time, within this time, Diavolo cannot interact with those outside of his own court of the crimson king, allowing virtually anything pass through him. Once the effect ends, the ones within range would have done what they were going to, but don't remember doing it which includes movement or death. However, the erasure of time would've been very hard, as you would need to predict what your target would do and avoid it or utilize it to be effective with King Crimson, and thus brings us to his tumour, Epitaph.
Epitaph is a small face that resides on King Crimson or the user's forehead, and has no offensive options. It's main ability is to allow someone to see up to 10 seconds into the future by looking into their hair, in which the visions will almost always play out exactly as they are shown. If you were to see that you are still alive after an oncoming attack his happening, you WILL survive and the same goes for death. Due to it not having anything else, the visions usually rely on the user's own skill to come true, or lack thereof.
King Crimson and Epitaph can be used simultaneously, which gives Diavolo a major advantage. Epitaph would predict what would happen, King Crimson would then Skip Time and allow Diavolo to avoid an attack and strike from behind, or King Crimson would erase time, see the visions of Epitaph and reposition himself in a way sot that he would always be the victor.
Long story short, if you want a sub-stand, make sure you make them play off each other and in doing so, create the most powerful stand(s) in the part.