Big Boned:  The Beast's size is large

Amphibious:  The beast can breathe air and water. It also gains a swimming speed equal to it's walking speed

Sneaky: The beast has expertise in Stealth

Relentless Rampager: When the beast is reduced to 0 hit points but not killed outright, it can drop to 1 hit point instead. It can't use this feature again until you finish a long rest. Additionally when the beast reduces a creature to 0 hit points with a melee attack, the beast can use its reaction to move up to half of its speed and make one attack. If it hits with that attack, it gains half of the damage dealt as temporary hit points.

Soul Infused attacks:  your companions attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Companions protection:
It gains proficiency in all saving throws. Additionally, it has advantage on Intelligence and Charisma saving throws when you are within 100 feet of it and it can see you.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 5) piercing damage.

Claws. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 5) slashing damage.

Chaos Bolt (1/day). Once a day as an action it can Chaos bolt using Constitution as it's spell casting ability +6

Fire Bolt. As an Action it can cast the cantrip Fire bolt using Constituion as it's spell casting ability +6    

Revenge. The companion can use its reaction to make one attack against the enemy if it is within range.
Roar. It uses its reaction to let out a frightful roar to impose disadvantage on the attack roll as long as the attacker is within 30 feet of it and is not deafened.
Body Shield. If your companion is within 5 feet of you when the attack is made, it can use its reaction to redirect the attack to target the companion instead.