Spells needed to be listed in the spell_index.json to be registered in game

Effects run top to bottom in that order.
[
  damage,
  poison,
  slowness
]

is not the same as

[
  slowness,
  poison,
  damage
]


this matters for 
-conditional effects
-chaining logic
-combos

Effect stacking does not work
-Minecraft will refresh or override the effect
-not stack separate poison instances

That’s vanilla behavior, not your framework.
