Multiple Sclerosis (MS) is a progressive disease of the central nervous system associated with long-term disability. Rehabilitation in both the cognitive and motor domains is key to comprehensive treatment of the impairments of MS patients. Since the effectiveness of rehabilitation is related to the levels of patients' motivation and engagement, there is an urgent need to develop strategies to keep patients engaged during rehabilitation. In recent years, several serious games in Virtual Reality (VR) have been developed for rehabilitation purposes, holding promise to enhance traditional approaches with gamified and entertaining environments. These technologies pave the way to increase motivation during rehabilitation sessions and thus promote training effectiveness. This paper presents an immersive video game that transfers traditional rehabilitation exercises into a playful and engaging context. The game combines Upper Limb (UL) motor activities with cognitive tasks and suggests different exercises targeting a variety of rehabilitation scopes. A biofeedback system based on heart rate data is integrated into the game to adapt the difficulty level of the game to the player's condition in real time. The aim of this preliminary study is to evaluate the usability of the developed game and to observe the effects of the biofeedback on the player's engagement during the game. The main finding of this study is that the biofeedback significantly increases mental focus on the game tasks and thus has a great potential to improve engagement in the rehabilitation session. The game is very well-accepted by healthy subjects and by one MS patient, suggesting its potential suitability for use in rehabilitation settings. Future studies will involve a larger cohort of MS patients to further evaluate its relevance and appropriateness in rehabilitation trainings.