I'm a SL11 and managed to invade a guy in undead parish who was wearing full Havel's and carrying Black Knight Sword. I get where he got Havel's from, but how can a 11 +-10% use the black knight sword? I don't get it. Did I invade a way higher level character?
If the Black Knight Sword was wielded in a two-handed fashion, the Strength requirement is lowered to 14. With the Dexterity requirement being 18, it is possible for a Pyromancer to have this equipment set at level 12.
However, if you are the one invading, you can invade players of any level. While the Dark Souls system attempts to match you with players that are around your level, you will eventually work your way up to high-level players if there's no-one else around.