https://kbromwich.github.io/dsim/
Performs simulations of attacks based on custom definitions to produce damage statistics (mean, stdev, min, max). Much of the expression syntax is specifically for D&D 5e. However, with some extra work you might able to use it for other systems.
Simulations are defined with the following form:
<name>@<level(s)>: <expression>
Where name is the name of the "build", level is the player character level, and expression defines the simulation to perform.
For example:
GreatAxe@1: 3+PB =atk> 1D12+3
Dual Wield@1: (3+PB =atk> 1D6+3) + (3+PB =atk> 1D6)
GreatSword with GreatWeaponMaster@1: 3+PB-5 =atk> 2D6+3+10