Forbidden Portal (MTG)

[NeoRWBY65]

[Mysty]

Forbidden Portal
UUUUBBBB
Enchantment
When you control no creatures, put a charge counter on Forbidden Portal, then put X X/X Portal creature tokens that are blue and black onto the battlefield, where X is the number of charge counters on Forbidden Portal.

^This is how it should be written on your card:
• a card referencing itself usually uses its own name, with a few conditions creating exceptions. This is not on of those cases.
• The token ability I wrote similar to Hazazon Tamar. Multiple tokens with multiple colors gets ambiguous.

As for rating the card itself - Note that I'm not a mind reader, so I can't tell what the original point of the design was; I'm just going to look at how I would have designed this card if I were given this sort of design:

At this much mana and that specific mana, it costs too much to be used in anything other than infinite sacrifice fodder for a combo. Personally, I'd do my best to fix both.
First, I'd make it unable to be used in an infinite combo, by making it trigger at the beginning of a phase or step. For this example, I'll say upkeep:

At the beginning of your upkeep, if you control no creatures, put a charge counter on Forbidden Portal, then put X X/X Portal creature tokens that are blue and black onto the battlefield, where X is the number of charge counters on Forbidden Portal.

Now we need to do something about that cost. It doesn't need to be all color-specific, and that's probably the first thing I'd change. To keep it those colors, let's change it to 4UUBB. Now its mana cost is manageable but it's still prohibitively cost. I think I could possibly see this card as a 6 mana enchantment, as cost either 2UUBB or 4UB (I'll choose 2UUBB).

With the end result being:
Forbidden Portal
2UUBB
At the beginning of your upkeep, if you control no creatures, put a charge counter on Forbidden Portal, then put X X/X Portal creature tokens that are blue and black onto the battlefield, where X is the number of charge counters on Forbidden Portal.

[NeoRWBY65]

Thank you for the corrections on my wording of the effect. I'm going with your suggestions, but I'm also going to give it a second effect. The second effect, in my opinion, should make it worth 8 mana.