It must first be said that Marius possess immense power for a number of reasons.
Number one, he is old. After two thousand years, Marius has grown stronger with every year as every vampire does.
Number two, there is a good chance that Marius' maker had never made another child before. We know that with each child that a vampire makes, each become weaker and weaker in terms of their vampiric strength. Since Marius' maker was a God and there can only be one God, there would have been no reason for him to make another.
Number three, Marius drank frequently from Akasha from the beginning of his vampire years on. Drinking from the very blood source itself increased his strength greatly. But, that was the intention. Akasha knew that Marius had to be stronger than all other vampires to protect her.
Number four, Marius has suffered physical catastrophe. For example, after he was burnt by the Roman coven, Marius found himself a great deal stronger when he had healed. So, obviously being hurt makes a vampire's body stronger.
The Mind Gift
The Mind Gift is a very intricate power, one that not many immortals have or realize their true potential in. Within the category of the Mind Gift, Marius has the ability to read thoughts as they happen, probe the mind for things within the subconscious, select images, send thoughts of his own into minds, move objects, and cloak his mind from being probed entirely (B&G pg. 165-167).
The Fire Gift
Marius first discovered this power when he was 'battling' Eudoxia (B&G pg. 163). In his anger, he wished one of her boys burnt, who promptly burst into flames. Marius practiced and discovered that his power had no limit to space, meaning it doesn't matter how far something is from him (B&G pg. 165). Marius does this again to Eudoxia, burning multiple vampires at the same time (B&G pg. 180), so he is clearly capable of sending out that force in multiple directions with accuracy.
The Cloud Gift
This was one of the later powers to come to Marius, or rather, one of his later powers to realize (B&G pg. 208). With this power, he is able to think of a destination and move there faster than the eye can see.
The Killing Gift
Marius didn't even know that he possessed this one until he used it on Eudoxia (B&G pg. 163). Using it, he repelled her attacks and knocked her down to the floor, causing enough pain in her when he lashed out that she lost her strength and would go weak. Marius described this power as pressure (B&G pg. 165). Testing the power out later on rats, Marius determines that the power would explode a mortal from the inside since the rats had exploded (B&G pg. 166).
The Healing Gift
A power to heal fast. After his first time drinking Akasha's blood after the incident with the Elder, Marius goes home and drives a knife through his hand, witnessing it heal over completely immediately (B&G pg. 83-84). The same went for after he was buried by ice in QotD. As soon as he stepped out of the ice, his body began to heal and his bone mend.
The Speed Gift
Related to the Cloud Gift, perhaps, but Marius can move faster than the eye can see. He seems to vanish and reappear.
The Spell Gift
This power is used to hypnotize and put a 'spell' over mortals. Marius uses this a lot to gain the trust of his slaves, putting a spell on them or coaxing their mind into believing something (or disbelieving something). It is how he gets what he wants or escapes suspicion.
- In addition to the powers above, Marius does possess a lot of strength. He can open doors impossible for even a team of mortals, carry heavy caskets up mountains... he even threw Mael across the room with one easy push (B&G pg. pg. 112). Even if he doesn't bust out the preternatural powers, most (about four immortals out there) would still have no chance against him hand to hand (though Marius has expressed his dislike for hand to hand combat).