I have over 500 hours of gameplay on Paladins and I have 8 levels on Corvus, so I know how the game works and I have tested him out in various situations. The only things I didn't test out is him playing damage (with the Stunning Visage) and I'll give my reasons for it later on.
Let's get to it.
I like to play support and play them very frequently, so I do have some knowledge how other supports perform in comparison to Corvus. In my opinion, he is underpowered. Not to the level of being useless - he performs rather well in company of other support. However, I believe he could use a small, harmless buff to his legendaries and one of his cards in order to make him a more viable support.
First of all, I think that Corvus is well balanced with his general kit. Marking people means he always needs to know where his allies is and can pull out a lot of tricks with it. His damage is alright and I have no complains about it, though, as I said, I didn't play him as a damage.
Now, why do I believe his legendaries need a buff? From what I've read here and there, Spreading Influence is his best legendary... Which, I believe, is complete rubbish. Let me explain.
The only thing this legendary gives me are elimination kills (with Kill to Heal item it does provides some sustain in tough situations). But it is not enough. The penalty is so big that healing marked champions is almost impossible when they are being attacked (especially when Cauterize is in use). I have maxed out both lifesteal and heal buff cards and it still doesn't do much. Honestly, the only situation in which this legendary proved useful when I had marks on two people on the point and I could get a bonus healing on both of them - when healing them personally, of course.
Dark Gifts, on the other hand, are alright. They don't do much, however. The bonuses are nice, although hardly visible, but when compared to Spreading Influence... I was literally panicking while using Influence instead of Gifts, because my healing was insufficient. What is bugging me, though, is that 50 points heal every 1 second. It's... not enough.
As I said a few times already, I didn't play Corvus as damage. For two reasons. His "damage" legendary seems a bit suicidal for me. In most cases, I would have to teleport into the enemy team to deal the additional damage. It could come useful when dealing with a flank (that's 1000 damage and a free stun), but it would come with a heavy price. I don't see Stunning Visage being better than Spreading Influence or Dark Gifts. Also, he really doesn't have much options for a good dmg builts (although I'll admit, it takes me a few takes to make a functioning loadout).
And last but not least, his cards. They are... they are not bad. But... They are not perfect. I just can't imagine using some of them instead of others. But one card in particular... Makes me grit my teeth.
When I was making my loadout for the first time, I saw Abbysal Effluxion and thought: "This is a good card." I got it to lvl 3 and never looked at it at that time. And you know why? Because I thought I was leveling up the damage reduction. Not the DURATION. Then I came back... and, oh boy, I raged. 5% damage reduction is nothing. Especially for such a short amount of time! It's like lending Furia a sunscreen before she gets roasted by Fernando! Without applying it on the skin!
Oh boy. Alright. I've raged a bit. Let's get to my ideas. Which will get ignored, but I want to have it out of my system.
Idea nr 1: 2 marks, 8% healing penalty - it still will be painful, but this will at least allow me to heal marked people a bit
Idea nr 2: 2 marks, 10% healing penalty, 50 healing per second - bigger price but with some cherry at the top
The only change here is that I would made the passive healing bigger - up to 100 points per second.
EDIT. I got an interesting idea to add the passive healing buff to the Mark as a part of the kit. Just like they did with Jenos' mark. Just an idea, though.
Damage reduction 5|5% for 1s.
On the whole, Corvus feels like a champion that they really didn't want to make OP. And they succeeded. A bit too much, in my opinion. But hey, it's my opinion. Feel free to post your own views.
TL:DR - Corvus' Dark Gifts should heal for 100 per second, Spreading Influence has too big penalty on healing, and Abbyssal Effluxion card is useless. Otherwise, he's quite balanced in my opinion.