General Update
Arrows no longer trigger Lightning, nor inflict Curse or Poison;
Lightnings no longer inflict Curse or Poison;
Star Allies no longer trigger Lightning, nor inflict Curse or Poison;
Lightning V: Your lightnings target your closest enemies (swapped with Lightning VII);
Lightning VII: Your lightnings deal damage in an area of effect (swapped with Lightning V);
The affected upgrades have updated descriptions to explain the new interactions.
New Upgrades
Lightning Arrow X: Your arrows can also summon lightnings;
Poison Arrow X: Your arrows can also inflict poison;
Curse Arrow X: Your arrows can also inflict curse;
Poison Lightning X: Your lightnings can also inflict poison;
Curse Lightning X: Your lightnings can also inflict curse;
Lightning Star X: Your star allies can also summon lightnings;
Poison Star X: Your star allies can also inflict poison;
Curse Star X: Your star allies can also inflict curse;
All the above upgrades need their respective upgrades on maximum to show up; i.e: for Lightning Arrow X to show up, you need both the Arrow VII and Lightning VII upgrades equipped.
A note from the Dev
These changes were made to better clarify how the mechanics work while also adjusting them to feel more fair.
Before these changes you could pick the Arrow V and Lightning V upgrades and just be invincible for 80 waves straight, due to free chained synergy between them.
Now with this update, these interactions will still be possible, but will be more difficult to achieve, needing a whole build around them for the upgrade that merges them together to appear, leading to a tougher early game that can still be facilitated through meta progression upgrades, more interesting builds and decisions, and not just pick-everything gameplay.