Epic is currently distributed under the GNU Affero General Public License, version 3.

This mean that it can be freely used in any open source project distributed under such a license.

The GNU Affero GPL impose the wider open-ness to any depending code: any application built upon Epic MUST provide a link to the full source code of the application itself to all it's users, even if the user consume the software through the Software As A Service business model.

On the other hand, as far as you follow the Epic's design and development constraints, your domain model will not depend upon Epic itself, so that you will be able to distribute and or sell it with any other (even proprietary) license.

As for the application built around the domain, contact the authors to obtain a linking exception if you can't accept the terms of the Affero GPLv3 (for example because you want to develop a software that will not be available to everybody on a well known publicly accessible http address).