She had picked out a costume for herself several days earlier (a modest Grecian chiton, to match her twin) and was just arranging her hair when Thalia had burst into the apartment urgently, two costumes slung over her shoulder.
Apparently the muse had taken Apollo's concerns regarding his sister's need to "lighten up" to heart. As she told it, she had been shopping around for her own outfit when she had encountered one which had "instantly reminded her" of Artemis. The huntress had eyed the pleather dubiously. She had a sneaking suspicion that Thalia was having her on.
But then Apollo had emerged and suddenly it had become two against one, and with a lot of cajoling and wheedling brother and half-sister had convinced Artemis to "just try it on". "You don't have to wear it out if you don't want," Thalia had pressed, "Just see if it fits!"
Of course it had been a slippery slope from there. The moment she had stepped out of the bathroom the other two had showered her with approving words.
"Sis, you look hot," was Thalia's appraisal.
Artemis had scowled. "I look ridiculous."
"Bullshit. You look ready to kick some ass!"
"In stilettos? I'm more likely to fall flat on my face. And who the hell wears shoes like these on a hunt?"
"It's fantasy, babe, we're not striving for realism."
"And a cape. 'Cause those are real practical in the forest."
It had all culminated into one of them luring her out into the hall on some fabricated premise, upon which they had slammed Artemis' door shut, leaving them all locked out with the key on the inside.
"Guess we'll just have to go as we are!" Thalia had chirped unapologetically.
And that's how Artemis came to be boarding Death's yacht in skimpy warrior-woman attire, silently plotting to kill Apollo when all this was over.