A Void Portal is a portal that takes you to the Void Zone in Forager. It got added in the Combat patch, so you won't find it in the base game. However, if you have updated to play on the latest version, you should be able to get it.
There are several ways to get the Void Portal. The first method is to obtain the skill called Sigil Maker. This is found at the very bottom right corner of the Skill Tree. When you have this, you are able to craft a Void Portal.
Another method is to look for the Travelling Merchant. He sells Void Portals, and you can buy as many from him as you need. Most players find this to be the easiest way to get them, but it's really up to you.
Some players have reported seeing Void Portals on the Market at well. The Market sells a random variety of items, so this seem possible, but I have not been able to confirm this for myself.