You will need the help of 3 witches. Two of them are Helda and Mercy. The third witch can either be Mary Warren or Grace, with Mary Warren probably being the easiest to get.
1. Get your Titan Time Travel power back at the Beginning of Time. It's fairly easy.
2. Go to the church in Salem 1969 and try to get in. If it's not open yet just wait around for a while until it opens. Or you can go to the high school -- OOPS I MEAN "COLLEGE" -- and mess around with Sarina's friend for a bit. Don't mess around too long or the ceremony will be over. Once you get into the church you'll get killed by Father Blackmoon but you'll retain the knowledge that you need three witches to help you. Rewind time.
3. Travel to 1692. Mary Warren's starting position is in the bottom-left of Salem 1692, so if you rewind time and head in that direction you'll have the best chance of finding her. Talk to her multiple times, even though she's attacking you. Eventually you'll "capture" her and she'll be sent to Castle Kronos at the End of Time. IMPORTANT: Rewind time now in order to reset Mary Warren. You'll still have an alternate version of her at the castle.
4. Travel to The End of Time. Talk to Mary Warren in the courtyard tile next to the castle. She'll give you a quest where you will need to talk to her in 1692 and give her some information. In exchange, you'll get her spellbook. Take it to Mary Warren in the End of Time and you will unlock Mary Warren as a companion. Set her as your companion from the wardrobe menu (talk to your daughter -- OOPS I MEAN YOUR "ASSISTANT").
5. Travel to Salem 1692 and rescue Hilda. She's being burned at the stake on the bottom left area of the map. Look for the weird color trees.
6. Rescue Mercy from town square. It's one of the village tiles. You can kill everyone except for the salesman in order to free her. Just find a rock in the same screen and bash the lock with it.
Note: Make sure you DO NOT rewind time in Salem 1692 or Helda and Grace will be captured again and you won't be able to use them at the church unless you rescue them again. If they don't show up for some reason then you need to go back to 1692 and rescue them.
7. Go to back to the church in Salem 1969 and try to get in. If it's not open yet just wait around for a while until it opens. Now you should have three witches to defeat Father Blackmoon. Watch an incredible horny and erotic sex scene with your character and "Sarina the 18+Witch" (your age may vary) then you'll get the Soul Steal Titan Power!
8. Now you can capture many female characters during combat by completely stripping them then clicking on the girl and choosing the power from the hex icon.
ALTERNATIVE: If you already have Grace or want to use her, you don't need Mary Warren. Instead, go to the hippie van in San Francisco and talk to Grace. Keep talking to her and her sister will show up. Complete Judy's quest and you'll have the option to take Grace on a date. This will send Grace to your castle and instead of setting Mary Warren as your companion you can set Grace as your companion and use her during the ceremony to get your Soul Steal power.
Hope this helps!