Venamis is an incredibly obscure character with little real characterization or description due to his own lack of appearances, and is not very often, if at all, used on the boards. However, his showings do have a level of merit and his duel with Plagueis is often used as a dueling feat for Plagueis since it is Plagueis's only known dueling feat. There are no canonical images of Venamis, and I have little respect for him anyways. I just want to convey his feats.

Power

Venamis' power is nearly equal to Darth Plagueis' as of 67 BBY:

The Force had whispered to him: Your moment has come. Claim your stake to the dark side. Act now and be done with this. But the Force had only advised; it had neither dictated his actions nor guided his hands. That had been his doing alone. He knew from his travels with and without Tenebrous that he wasn't the galaxy's sole practitioner of the dark side -- nor Sith for that matter, since the galaxy was rife with pretenders -- but he was now the only Sith Lord descended from the Bane line. A true Sith, and that realization roused the raw power coiled inside him.

And yet...

When he reached out with the Force he could detect the presence of something or some being of near-equal power.

Source: Darth Plagueis

Combat Skill

Darth Venamis is an enthusiastic combatant and enjoys lightsaber duels:

To Plagueis, lightsaber duels were tedious affairs, full of wasted emotion and needless acrobatics. Tenebrous, however, who had pronounced Plagueis a master of the art, had always enjoyed a good fight, and had clearly bequeathed that enthusiasm to his other trainee.

Source: Darth Plagueis

Venamis can use multiple forms ambidextrously, and Plagueis is apparently surprised by his skill with a blade:

For no sooner had the blades of their weapons clashed than Venamis began to bring the fight to him in unexpected ways, twirling his surprisingly limber body, tossing the lightsaber from hand to hand, mixing forms.

Source: Darth Plagueis

Venamis is able to fight while suspended in the air, as if on high ground, which is also a testament to his combat skills and their synergy with his Force abilities:

At one point he leapt onto an overhanging greel branch and, when Plagueis severed it with a Force blow, hung suspended in the air—no mean feat in itself—and continued the fight, as if from high ground.

Source: Darth Plagueis

Venamis was trained to become an expert in Darth Plagueis's style by Darth Tenebrous, allowing him to not only present a challenge to Plagueis, but also briefly penetrate his defenses. Only when Plagueis revises his strategy is Venamis beaten:

Worse for Plagueis, Tenebrous had made Venamis an expert in Plagueis’s style, and so the Bith could not only anticipate but counter Plagueis’s every move. In short order, Venamis penetrated his defenses, searing the side of Plagueis’s neck.

Source: Darth Plagueis

Speed

Venamis fights and runs so quickly that his contest with Plagueis would have looked to an onlooker like lightning flashing through the forest's understory, which is even more impressive when one considers the unfavorable terrain:

The contest took them backward and forward through the trees, across narrow streams, and up onto piles of rocks that were the ruins of an ancient sentry post. Plagueis took a moment to wonder if anyone at the fort was observing the results of the contest, which, from afar, must have looked like lightning flashing through the forest’s understory.

Source: Darth Plagueis

Concealment

Venamis is able to able to succeed in traveling to the surface of Sojourn undetected. Plagueis himself cannot sense the dark side in him, only the Force, which is a testament to Venamis's own potency with Force Concealment:

A Jedi spy? he wondered.

They had tried repeatedly to penetrate Sojourn’s defenses during previous Gatherings. But unless one had arrived in a ship designed and built by Darth Tenebrous, there would have been no way to reach the surface undetected. And yet someone had obviously succeeded in making it downside. Lifting his hand from the hilt of the lightsaber, Damask minimized his presence in the Force, surrendering his eminence and disappearing into the material world. Then he began to move deeper into the forest, winding his way through the trees, allowing the Jedi to stalk him even as he berated himself for having acted rashly. If it came to ambush, he would not be able to fight back and risk exposing himself as a Sith. He should have allowed the Sun Guards to deal with the intruder.

But why would a Jedi bother to trip the perimeter sensors only to retreat beyond their reach? They didn’t make mistakes of that sort. And surely whoever was out there wouldn’t have expected a Muun to respond, if for no other reason than Muuns didn’t make mistakes of that sort. So what was this one after?

Ahead Damask heard the characteristic hiss and hum of a lightsaber, and saw the weapon’s blade glowing in the mist. Emerging from behind a thick-boled tree, the wielder had the lightsaber in his right hand, angled toward the spongy ground.

A crimson blade in a crimson wood.

Instantly he called his own lightsaber to his left hand, igniting the blade as the figure in the mist revealed itself fully: a tall, thin, pink-skinned craniopod with large lidless eyes—

A Bith!

Tenebrous?

He faltered momentarily. No, that wasn’t possible. But who, then? Tenebrous’s offspring, perhaps—some spawn grown from his genetic material in a laboratory, since the species reproduced only in accordance with the dictates of a computer mating service. Was that why Tenebrous had declined to discuss midi-chlorians or ways of extending life? Because he had already found a way to create a Force-sensitive successor?

“I knew I could draw you out, Darth Plagueis,” the Bith said.

Plagueis dropped all pretence and faced him squarely. “You’re well trained. I sensed the Force in you, but not the dark side.”

Source: Darth Plagueis

Force Flight

Venamis is able to utilize a limited form of Force Flight mid-combat, enabling him to fight while suspended in the air, as if on high ground - a feat that somewhat impresses Plagueis:

At one point he leapt onto an overhanging greel branch and, when Plagueis severed it with a Force blow, hung suspended in the air—no mean feat in itself—and continued the fight, as if from high ground.

Source: Darth Plagueis

Lightning

Venamis is capable of producing a Storm of Force Lightning, but stops his attack as Plagueis crosses two blades at his neck:

With a gesture of his other hand, Venamis called for his lightsaber, but Plagueis was a split second quicker, and the hilt shot into his own right hand. Sensing a storm of Force lightning building in the Bith, he crossed the two crimson blades in front of him and said: “Yield!”

Venamis froze, allowing the nascent storm to die away, and dropped to his knees in surrender as Sojourn’s risen primary blazed at his back through the trees.

Source: Darth Plagueis

Telekinesis

Venamis could have pulled his lightsaber to his hand with simply a gesture, but Plagueis was a split second faster, which allowed him to summon the blade to his hand first:

With a gesture of his other hand, Venamis called for his lightsaber, but Plagueis was a split second quicker, and the hilt shot into his own right hand.

Source: Darth Plagueis