Complete the game and load your cleared saved game file to start with all levels, items, and stats from your previous playthrough. You will also get the Triple Rune, and the game will become more difficult.
Once you defeat Leonard and get to the scene where the Diva of the Twilight threatens to kill the Wind Sage, keep the wand. You will become evil Isaac under Diva's control and can roam around to collect the lost spells for evil and let Diva win. However, if you give up the wand, she goes away leaving your friends alive, letting you continue. If you follow the pointers, you will find another wand. No matter what, Diva of the Twilight will still make her new castle.
After showing the Master of the Curtain Mist what you are made of and receive one of the Lost Magic, travel to where he is continuously (4 times) without any other battles. You have to have 2 Runedines with you. You have to get incredibly lucky. For those of you who are looking for a purple Runedine and a challenge, this is it.