It's simple, after allocating every major (circular) node on a character tree, a new major node appears, close to the first node for the character. The said node is very game-changing, and will need 5 skill points to allocate. The effects should be simple, but still have a large effect.
Here are some of the ideas I came up with: Magdalene: Start with 9 Volt and a gulped AAA Battery. Cain: Stealing is no longer randomized, hold drop to steal something, 5 times per floor. Samson: Every hit you take also gives you a permanent +0.1 damage. Azazel: +1.5 base tears, +5 range. Keeper: 10% chance to drop Soul of the Keeper when using Wooden Nickel. Forgotten: Makes the chains longer, letting the soul travel further away. T Samson: 150% Longer Berserk duration and +50% size buff.
Those are all for now!