Ruler Of Kings 2 Text RPG General Discussion - Page 150
Natural D20 added to Combat. What this means is if you roll a "20" on a D20 but still are not able to penetrate the Enemy's AC then you still score a 1HP hit.
Beware, the Enemy will also have this opportunity.
Later, when I develop specific Character Class bonuses then the D20 will act like a bonus as well for additional damage to the Enemy.