Ok the rules seem a little light when it comes to runes so we've used what runes are in other sources to make super powerful weapons with many runes attached to them.

So is a rune a weapon in itself?

Or is it an other with a limit of two thats attached to a magical weapon?

Or can you just sit there stacking up runes and rolling almost every dice in the game?

All Runes also have another item type. For example, Immolation is a "Weapon - Rune" (and has 2 hand icons on it), while the misleadingly-named Ghost Armor is an "Other - Rune". They follow all of the normal rules for whatever type of of item they are other than a Rune, and the fact that it is also a Rune normally just means that you can't use it if you're wearing Chainmail or certain other armors.

So, for example, equipping Immolation uses up both your hand slots. You can attack with it, because it's a Weapon. If you have multiple weapons equipped (which isn't normally possible when they're two-handed), then each time you make an attack, you choose one weapon to attack with, and the others don't provide any benefit to that attack (except possibly an Off-Hand Bonus, but that only applies to melee attacks).

Thanks very much that really clears it up.