How to make mobs have a random sword?

1. Make a standalone procedure (name it as you like)
2. Set global trigger to: Entity Spawns
3. Add an "if" block with target entity (sub)type of <[desired_entity]>
4. Add a random block 0,1 ... with a (0.1), this (the example) will give you 10 options
5. Add "set item in main-hand" block for each option
6. add the disired items/weapons (1 each)
7. Save and test

EXAMPLE FOR 10 OPTIONS:

Forgot to mention, it will be a random weapon in hand every time this entity spawns.