Knockdown Drywall Knife
You need a knockdown drywall knife to trawl your texture. There are plastic (on the right) and metal blades (on the left).
If you are applying a hand brocade, you can use both, but the stiffness in the metal blade is needed to get a clean edge (where the ceiling meets the walls).
The plastic blade is great for sprayed knockdowns. You can trawl the ceiling texture earlier than the metal blade. If you get on it early with a metal blade, you will smash the texture.
In addition the the knockdown knives, you will also need some drywall taping knives. The standard knife everyone should have is a 6" knife.