Originally done here.

So, this is the thing in Ob Nixilis, the Fallen's forehead. Apparently, he was a user of the Chain Veil at some point, and was beginning to turn into a Demon because of the same curse Garruk has now. He eventually ended up in Zendikar, where a female kor named Nahiri (recently confirmed to be the Lithomancer thathelped Ugin and Sorin trap the Eldrazi originally) suppressed his transformation and power with this very artifact. In Duels of the Planeswalkers 2015, your avatar takes this hedron from Ob Nixilis to use against Garruk, and succeeds in slowing his transformation, but not curing the curse completely. Which leaves us with Ob Nixilis, Unshackled quickly recovering his former power.

Why not just have it read "Activated abilities of planeswalkers can't be activated?"

This way gives the ability to safely grow the loyalty while it can't be attacked; then remove this and activate the ultimate. "Go, do this" hints are a useful thing.

But yeah; "Planeswalkers are just creatures now" would be cleaner.

It's in addition to their other types. So they can still be attacked. Allowing them to still activate increase their loyalty makes them not die immediately. That said, it may more less intuitive than I thought. At the very least because right now I noticed a planeswalker creature can block for itself.

I'll try removing the planeswalker type and just preventing loyalty abilities.