This is my personal build with Terminus, I've found a LOT of success with it. It requires a lot of gamesense and predicting your opponents, but if you can, he can be a monster.
Necromantic Might and Abomination are at 5. This is so that you can get as much siphon as possible by hitting your opponents with Calamity blast.
Devastation is there because you will be shooting so many calamity blasts, might as well get more from it. Powerslave and Hulking Monstrosity are filler cards. Some other good options as filler cards are We Can Rebuild Him, Wrecking Ball, It Watches, and It Waits.
The filler cards you use will mostly depend on personal playstyle and what you might need/not need in a specific match. (I would highly recommend Powerslave though, as you're gonna have your siphon up a lot, so being able to move a bit faster with it can make words of difference.)
I chose Decimation as the legendary card, as you are going to be spamming your Calamity Blasts, so why not deal extra damage to them? (Sometimes, I can rack up almost as much damage as damages)
Nimble is practically a must-have for Terminus at all times, so take that. I personally like taking chronos. It sounds weird, but remember that siphon and calamity blast are actually both on cooldown. This allows you to bring up and down your siphon faster, allowing for even better siphon management and siphon baiting, and allows you to activate Abomination and Devistation even more.
Past that, the items vary from fight to fight. Generally though, I use resil, rejuv, bulldozer, and haven. (idk if rejuv is even good anymore, I just do use it)
I would not recommend caut or wrecker, as calamity blasts cannot apply it. Unless the enemy tank/shield is constantly in melee range, it's not gonna do much. I wouldn't recommend Master Riding either, as if you are good, you can avoid dying, plus you have revive on the off chance you do die. If you are dying a lot though, maybe Master Riding would be good for you.
To play this Term build effectively, you are going to need a lot of gamesense, so that you can predict when your opponents will try to shoot at you, baiting the shots into your siphon, good siphon management, and good projectile aiming, so that you can hit people consistently with your calamity blasts. (Missing a bunch of calamity blasts in a row can kill you)
However, if you can pull this off, Terminus will feel like an unkillable god with infinite siphon.
Each Calamity Blast deals an additional 100 damage.
Reduce the damage required to generate a Calamity Blast charge by 600.
Generate 10% Power Siphon charge for each enemy hit with Calamity Blast.
Heal for 180 for each enemy hit by Calamity Blast.
Reduce the Movement Speed penalty during Power Siphon by 15%.
Increase your maximum Health by 150.
Gain 7% Movement Speed.
Reduce the Cooldown of all your abilities by 10%.