Heroism points represent a character’s determination and their importance to the plans of the gods or the forces of fate (i.e., the game and the GM.) A character has a number of Heroism points equal to his or her level x 2 (or simply equal to level, for a less heroic game). Heroism points can be used to add to any attack, damage or stat + skill roll at a rate of +1 per 1 Heroism point spent. You can only spend up to your level in Heroism points on one single roll (for example, a level 10 character can only spend up to 10 Heroism points, for a +10 bonus).
Heroism points do not need to be spent all at once, but when they are completely spent, they are gone for the rest of game session. A character’s Heroism points are restored to their starting amount at the beginning of a game session. Alternately, the game master may restore 1 or more Heroism points on a die roll of 20 and/or if the character does something especially entertaining (funny, exciting, or appropriate to the character’s personality).
Magi can cast any arcane spell with aspell level equal or below 1/2 their class level, rounded up. They have access to all arcane spells in the SRD spell list. Casting a spell of any kind costs Hit Points. The cost is 1 + double the level of the spell being cast:
This loss cannot be healed normally but is recovered after 8 hours rest. There is no need to memorize spells in advance. Just because a character can cast any spell, doesn’t mean that they should. Choose spells that suit the character. Select one "signature" spell per spell level from 1st upward that they prefer to use over any other. These spells are easier to cast due to familiarity, costing 1 less HP to use.
The Difficulty Class (DC) for all spells is 10 + Caster Level + Caster's MIND bonus
Hit Points = STR Stat + 1d6/Level. If HP reach 0, unconscious and near death. Further damage directly reduces STR. If that reaches 0, death.
Roll d20 + DEX bonus for initiative order. On a combat turn, a character can take TWO actions, usually  move and  move again, make an attack, cast a spell, attempt another activity (requiring a stat + skill check), etc. Movement of 5 feet or less, drawing a weapon, speaking or similar activities are “free” and do not count as an action.
Melee attack bonus = STR bonus + Level
Missile attack bonus = DEX bonus + Level
Magic attack bonus = MIND bonus + Level
Add attack bonus to d20 roll. If higher than your opponent's Armour Class (AC), it’s a hit. Natural 20 is automatically a critical doing maximum damage.
Fighters and Rogues can use DEX bonus + Level as Melee attack bonus instead if wielding a light weapon. Fighters and Rogues can wield 2 light weapons and attack with both in a round if they take a -2 penalty on all attack rolls that round.
If the total bonus is +6 or more you gain the effects of the Spring Attack feat, i.e., you can split your movement on either side of an attack option.
Add STR bonus to Melee damage, x2 for 2-handed weapons.
Armour Class (AC) = 10 + DEX bonus + Armour bonus.
Falling : 1d6 damage per 10', half damage on Phys+DEX save. DC=depth fallen in feet
Spikes : add +1 point to falling damage per 10' fallen, max +10
Poison : Phys+STR save to avoid or for half, depending on poison.
Effect varies with poison type.
Extreme Heat & Cold : If not wearing suitable protection, Phys+STR save once every 10 minutes (DC 15, +1 per previous check), taking 1d6 damage on each failed save.
Leveling up comes at the GM's discretion, based on the pace that he wants the game to have. Personally, I prefer a pace of about 1 level for every 10 play sessions.
Each level adds:
+1d6 to Hit Points
+1 to all attack rolls
+1 to all skills
If the level divides by three (i.e. level 3,6,9,etc.) add 1 point toSTR, DEX or MIND.
Fighters gain +1 to their attack and damage rolls at levels 5,10,15,etc.
Magi gain access to new spell levels at levels 3,5,7,9,etc.
m20 D•H does not really support or encourage play above 9th or 10th level at the most.