Getting this trophy requires you to obtain a parasite and then use it to unlock your illithid potential. This comes in two parts: getting your hands on a parasite and unlocking the ability to use parasites. Parasites themselves are fairly easy to find, sometimes stashed away in bottles or taken from the corpses of True Souls. One of the easiest parasites to get your hands on early is from a dying True Soul near the Blighted Village as seen below (coordinates X: 143, Y:439).


Once you've obtained a parasite, it then becomes a matter of speaking to your dream visitor for the first time. This will happen naturally during a long rest, usually your first after entering the Goblin Camp or a forced long rest upon approaching act two of the game. Once your dream visitor tells you to make use of your potential, you'll finally be able to consume the parasites you've collected. Consuming one will unlock the illithid menu, where you can spend parasites on special skills. Unlock one skill on any character and this trophy will pop.