>>507882114Alright, I'll preface this beginning with the marketing phase of the DLC:
It begins with the first promotional image of the game showing Miquella riding on Torrent. This establishes the first thread of connection between them and the Tarnished, and is intended to show that they worked with Melina, Ranni and perhaps even Marika to ensure they could be strong for the journey ahead. We could treat this dismissively and say Miquella wanted to ensure the Tarnished could kill Radahn for his purposes, but it's important to establish this preexisting connection.
Lets dissect base game Radahn. He is stated several times to idolize Radagon and Godfrey, his armor is intended to be derivative of them melding their motifs. His cape has the symbol of the existing Golden Order. Why is this important? It showcases his loyalty to the existing Order and implies his desire to succeed it, which he demonstrates by holding the stars in place. An act that prevents any Empyrean from being able to interfere with and establish a new Order, as is shown during Ranni's questline. Knowing this recontextualizes Malenia's desperate battle with Radahn, as if she succeeded and he perished Miquella would be able to ascend to godhood in the Haligtree as Ranni did during the Age of Stars ending. It was a last resort and why the spirits of Cleanrot Knights and Redmanes fight so fervently even after death, it's the difference of whether or not Radahn gets to become the third Elden Lord or Miquella ascends to godhood and supplants the existing one.
Even if we say that part of the vow was that Radahn was to be defeated in battle by Miquella and Malenia before he joined the Haligtree Order, why would he already promise himself to a different Order by adorning their motifs and sigils, making clear his intent to succeed the existing Golden Order? From noticed this logical inconsistency and thus redesigned his DLC armor with the Haligtree Order symbol to be fitting.