1. Miquella wanted to enter the shadow lands.
2. Malenia serves Miquella.
3. Malenia fought Radahn.
4. Killing Radahn lets the player go to the shadow lands.
All together implies Miquella likely sent Malenia to fight Radahn so he could enter the shadow lands
I'm asking why?
The importance of Radahn to the main plot is that he's holding the stars back, and so, he's holding the destiny of the Carian royal family back.
So, what if St Trina is similar to a Carian princess?
We know alter egos can have completely separate lineages.
Marika was an Eternal and Radagon was a descendant of the fire giants.
So it's possible St Trina is related to Rennala or the Carian royal family.
And there is a pretty strong connection between Ranni and Miquella already too. Ranni knowing torrent's former master and even getting the spirit bell from him.
I’m pretty sure Miquella sending Malenia after Radahn (so that the stars could move for an “eclipse to enter a spirit world”) is literally cut content that was originally at castle sol.
Wasn't castle sol content just related to Godwyn?
Miquella was hoping for an eclipse to revive Godwyn or something.
And shadow realm seems to be not connected to the stars. It seems more like a dreamscape or a painting.
But maybe you're right. Maybe Radahn is gonna be way more important than what he was portrayed to be in base game.