Creating a home-brew creature for a 5th edition D&D campaign that will continue to be a threat to players throughout the campaign. The basic idea of the creature is that it is a creature of darkness that feeds and grows off life. The creature starts out relatively weak but each of its attacks increase its HP by a portion of the damage done. With no maximum hp. The amount of damage done is also proportional to the creature's current HP. As a result the creature starts out as weak and easily dispatched, but if ignored can become a TPK waiting to happen. 

Most of the rules found under the create a monster chapter of the DMG do not account for a creature that becomes more powerful as a fight progresses and as a result using the standard array for determining CR and XP value can be quickly invalidated. 

What method could be used to determine an Effective CR and XP value for such a creature?