Portal is a Legendary natural-type fruit in Blox Fruits. It is expensive and hard to obtain Portal fruit. It costs $1,900,000 or 2000 Robux and has a 7% chance of being in stock, so it will take some time to show up in a shop. It has a spawning chance of 6%, so there is a low chance that you will find it while playing.
Portal is one of the best fruits you can get, not because of the damage or the high PVP potential, but because it gives you a lot of mobility and travel. What is unique about this fruit is that it opens a portal allowing you to teleport from one island to another one. This traveling option is great if you are bounty hunting or trying to find other fruits. So if you have it, make sure that you use it as much as you can.
What combo you will use depends on your in-game situation, who you are fighting with, and what you want to achieve. It is best to spend some time testing all possible combos, and by doing that, you will learn how to use what works best for you and also how to counter it. So go and test Portal in a game.