looking for basic combat rules? here

Basics

Assuming you know how to roll a d20, and what advantage/disadvantage are.

Defense

AC and saving throws are different.

Everyone has a Defense score for each ability score (e.g. Dex Defense, Con Defense, etc). When attacking, the attacker rolls their attack/skill/power/etc vs a given Defense score. Physical attacks are typically against Str, Dex, or Con, as follows:

• Everyone has a Dex Defense equal to 10 + Dex
• If wielding a melee weapon, you can choose to force enemies to attack Str Defense instead, provided they are wielding melee weapons. Weapon size modifiers apply
• If you are wearing proficient armor, you can choose to force enemies to attack Con Defense instead. (If using monsters directly from D&D, you're probably assuming AC = Con Defense)

If attacking with a power, that power will say what kind of Defense score applies. For example, a power might read "Wis negates", meaning the power must beat Wisdom Defense to apply. If the power does not mention defense, assume Dex Defense, since the power needs to actually connect. (Also, tell the system designer not to be lazy)

If a power mentions that an ability check "ends" (e.g. "target burns for medium fire damage each round, Con ends"), then the target is entitled to roll that ability check vs the original power's DC once per turn as an immediate action in order to end the effect.

Only if a power explicitly says "no defense necessary to hit" does a power skip defense entirely.

What about player agency in avoiding deadly spells, traps, and breath weapons?

This system's philosophy is that Saving Throws were invented primarily to suit the above purpose, but have never been a good solution. A roll of the die is not agency--there is no choice to be made. Instead, this system provides various means of interrupting any kind of attack, using the Initiative system, or various reaction abilities. These have proven more interesting than rolling a die.

What about proficiency?

TBD

Proficiency

Every character has a "proficiency bonus". A roll without proficiency is 1d20 + relevant ability score modifier (per the DM). A roll with proficiency is 1d20 + ability score modifier + proficiency bonus.

Proficiency bonus is a flat +5 for all characters, unless otherwise modified. (The Hero class get improvements to proficiency bonus).

The above describes how proficiency applies to ability checks, skill checks, and saving throws. For weapons and armor, it's different.

Weapon Proficiency

Weapon proficiency is boring. Simply not having your +5 to hit with a weapon is irrelevant 99.9% of the time...fighters have all proficiencies, and non-fighters never use weapons.

Instead, weapon proficiency works differently, in order to deal with a problem that 5e had: multiple attacks per round.

Nobody gets multiple attacks per standard action in this system. Instead, weapon proficiency works as follows:

• No profiency means no proficiency bonus to attacks, as normal.
• Proficiency at level 1 means you get proficiency bonus to attacks, as normal.
• Every 5 levels, you gain an additional die of damage with proficient weapons. Thus:
  • A longsword deals 1d8 + Str slashing damage from level 1-4.
  • The same longsword deals 2d8 + Str slashing damage from level 5-9, 3d8 + Str from level 10-14, etc.
  • Note: no weapons in this system inflict multiple dice at level 1 (e.g. there is no weapon inflicting 2d6 damage; such a weapon is more likely to use 1d12 to better suit this rule)

Armor Proficiency

Encumbrance is boring. Spell failure failed. Dex skill penalties are redundant. Movement speed penalties are unrealistic and unfair. Armor categories are meaningless and arbitrary, and traditional D&D armor types are completely out of touch with reality.

Is there any room for Armor Proficiency at all? Maybe.

Instead of Armor Class--a term that was misleading and inaccurate decades ago, and is long since obselete--there is a DC required to hit any target with a weapon attack. That DC is 10 + Dex + size modifier (if any). This represents a target's ability to dodge attacks, and can be called a Dex Defense. Those with melee weapons can force melee enemies to attack their Str Defense instead.

With Armor Proficiency, you can force enemies to attack your Con Defense.

(Note: original idea was to add +Con to AC)