Very nice use of contrasting colors here, the pink appears to glow far better in the dark background. The little lines/fur effect around the gems gives the appearance of a murky ocean or power emanating, which is a nice touch. In the future, watch that the edges of some colors don’t blur too much. The very light grey you have surrounding some of the tentacle gives an odd effect when it overlaps the glowing areas, giving it an unnatural halo effect. The tentacles indirectly pointing to the center of the image gives the eye a focus, which is good utilization of artistic technique.
Being as we can’t see too many details due to the artistic style, the main portions seem correct. The tentacles are nicely detailed with the suckers, and the colors for the environment and pokemon itself seem correct. One point of anatomy that would have been good to add would have been the central smaller gem in the center of the forehead, it seems oddly absent when you have the larger gems and those creepy dots for eyes at the lower edge. The larger gems might also be a bit bigger.
While you didn't make the location of Tentacool painfully obvious, we can very easily assume where it is based on the slightest of hints that you gave. The lighting/shading and pokemon themselves suggest a very dark ocean, and its the fact that you hinted instead of trying to make it obvious that gives this image a lot of appeal. The tentacles seem rather well lit for such a location though. Perhaps the tentacles could have been tinted by the pink light emitting from the head gems in order to tie the theme together better.
1. You could have used the small gem in the forehead to act as a focal point to the image
2. Watch the halo effect on the right tentacle
3. Light effects on the scene, as with pink light altering the shading on the tentacles
The tentacle monster in training doth be yours, madame.