Once you get back from the Boyle Estate, go to the bar and you will see Callista. Talk to her, and she will ask you to find Emily, who is hiding from Callista for fun.

When I did this a few times, she is either down the hallway connected to your chambers, or she is up the stairs at the corner of the street.

Thank you to the person who put up the first comment* for mentioning another spot she likes to hide in, which is at the bottom of Emily's tower, in the kind of ruined section under the door farthest from Samuel.

If she is in none of these places, try finding her using Dark Vision, and put where she was in the comments if you'd be so kind.

Once you find her, she'll talk to you a little and give you the Rune.

*I didn't put their specific name on the guide because people can change names.

Now, before you say that there are alot more Runes the Heart cannot find, read a few of the first comments before you say anything. Thank you.

Now I hope this helps you get the powers you've been saving up for! If it did not, however, post in the comments below what went wrong or if something was slightly different.