Tacenda seemed to Planeswalk at the end. Did she, or did she just "ascend" within Innistrad? Was she an unsparked planeswalker before she accepted the Entity? If she wasn't, did the Entity give her a Spark, similar to how Slobad was grafted one? And if that's not it, the third possibility... is the Entity native to the Blind Eternities, like the Eldrazi?
I will give you both the canon and Brandon head canon.
Canon is: ambiguous on all counts. It's not confirmed that Tacenda planeswalked at the end, or whether she (or Davriel) had sparks of their own--or if their planewalking powers are granted by the Entities. I can say, in canon, that the Entities are not native to the Blind Eternities--they are the souls of ancient, powerful planes that were destroyed, and their power harvested.
The Soul of planes sequence from m15 were their inspiration, though something special happened with these specific planes. I can't confirm or deny how many of them there were, but at least two.
Now, the Brandon Head Canon. (See my post at the top of this thread for an explanation.)
In my discussions with Wizards, they gave me leave to create one planeswalker--but in the course of the story, I decided to have Tacenda spark as well. This wasn't done with canon permission from Wizards, and I promised them that I'd leave it ambiguous in case they didn't want to have to deal with me playing loose and free with creating planeswalkers.
I personally imagine that the entities are providing the sparks for both Davriel and Tacenda. Davriel THINKS it's his own spark, and it ignited when he saw the true nature of the multiverse--and this is, right now, the canon answer. But I personally like the idea that one of the reasons these Entities are important is because they provide planeswalking ability to whoever holds them.
That has too much lore implication for me to canonize, however--and I didn't push Wizards to do so. (I also didn't ask them to make Tacenda a planeswalker; I simply wrote the story, and left them the option of using her in the future.) So take this part all with a grain of salt.