This is in the context of a videogame. The complete quote is 「怒りのキーブレードは心を解き放つ」, which was translated into "The keyblade's rage releases those hearts". Why isn't it "The keyblade of rage releases those hearts"? What is the role of の here?
This type of の is found often in fictional works and adds a "symbolic title" to the modified word. Similar examples:
- 自由の女神 (The Statue of Liberty)
- 進撃の巨人 (Attack on Titan)
- 鋼の錬金術師 (Fullmetal Alchemist)
- 青の祓魔師 (Blue Exorcist)
- 鬼滅の刃 (Demon Slayer)
Official English translations are included in parentheses. As you can see, this construction is not always translated "literally" into English. If there was a weapon whose official name is 怒りのキーブレード, then it could be translated as "Rage Keyblade", "The Keyblade of Rage" or something like that. However that's not the case in your sentence. Since this 怒りのキーブレード seems to refer to someone's keyblade who happens to be in a fury, "the keyblade of rage" might sound too grandiose, and "the keyblade's rage" seems to be a possible translation to me.
Personally idk if it's possible to even translate that into natural English. 怒りの is not that different from say, 青いキーブレード, it describes what type of keyblade it is. Basically, it's "a keyblade that is used in rage/while the user is enraged" will unlock the door, as opposed to say, a "calm" keyblade, one whose owner is usually kind and not that angry, fights to protect rather than destroy, etc. I'm sure you've seen in anime where characters are like 'anger and hatred makes one stronger' or characters get emotional and then get a powerup.