That's it. The core 3 cards you need. Throw whatever you want for the last 2. Fade to Black, Sorceress, and Spirit leech (at least lvl 2 for SL) are all good picks.

Fade to Black is better than the flat cooldown reduction on Shadow Travel unless you have decided you never want to right click.

Veil is a crutch card, haven't used it in years. If you're having difficulty and are new to Seris, fine, use it as one of the last two slots, but try reducing it after playing for a while and you get comfortable, and eventually try to remove it entirely. You are beefy enough and have enough life steal (a % of your max HP heal with your Q) that you shouldn't be getting bursted down unless you made some other mistake.

Umbral Gait is a given, most other Seris builds get that one right. Whether it be peeking, repositioning, chasing, or getting distance from a flank, it's amazing. You should ALWAYS have at least one stack on an enemy, for the movement speed and for an emergency Q heal if needed.

Blood Pact gives you the sustain that makes this build so juicy. If you're getting poked, just keep healing. Don't stop healing. Don't be one of those losers who thinks "oh no I'm getting shot at I have to run and leave the rest of the team to die while I'm healing up!" Screw that, tank those shots head on. The most important part of playing Seris, especially with this build, is that you are never out of the fight. You are ALWAYS doing something, and never should be running away with shadow travel or retreating to a position where you're out of the teamfight.

Mwah, chef's kiss. The trifecta of items for Seris. First buy is always tier 1 haven, then either t1 resil or nimble depending on the other team's amount of CC. Nimble can be bypassed if the enemy team doesn't have good non-bubble shields so you can constantly have Umbral Gait up, in which case you can pretty much take your pick from the rest of the standard items. Caut (since you're good at spreading it with penetrating shots), wrecker (to deal with pesky shields), morale boost, etc are all viable as usual. Max out your haven first, get resil stacked if needed, and then just flex into what's in demand.

You're more to your team than a healbot, but this doesn't mean you lose healing value. You still should be spamming heals off-CD, but what you are doing between heals is what matters. You have enough time to fire 6 orbs between your heals, which coincidentally is exactly as much you regain with tier 2 spirit leech. At max soul collector stacks each orb will deal 273 damage, so those six shots alone will deal 1.6k damage to each enemy they pass through. After that, your heal will be up and you'll be back at full ammo after its done. If you are ever running out of ammo, you're doing something wrong. You are wasting time and losing value. This is the most infuriating part, to me, of watching other Seris players. They have their heal on cooldown and they look like a lost child in a supermarket.

You have more value than a right click, just use it.

You are an infinite ammo poke machine with just over 3k hp, good sustain, and near-permanent 40% extra movement speed. You have map presence since you can maneuver around with that speed mid-fight, and you can more easily get LoS on allies and enemies alike. Throw your orbs right down the point lane! Poke the enemy with orbs and then hold your Q to keep a source of healing in case you get into a fight.

Get flanked? Be confident. You have 3k hp, 910 DPS, and your Q heals you for 240 per stack you rend. You don't need someone to guard you from the flank, you can guard yourself.

"But isn't getting max stacks on soul collector hard?"

For some reason I see this question a lot, and it was even mentioned as a reason for reducing Seris' Q cooldown. I've never once had a problem reaching 15 stacks, hell I get half of them throwing random orbs towards the spawn path or objective at the start of the match. If the enemy has decent cover with shields, just get another angle or help burst it down. All you need is around a one-second window to get four orbs on someone, as well as anyone behind them.

"How do I get healing/damage with Q while maintaining Umbral Gait benefits?"

A lot of the time I see people not getting the Umbral Gait speed buff. The way to use it correctly is to throw your orbs and then Q right before the last one lands. Once you get used to the travel speed this gets easier, and if you're really having trouble, just throw one last orb after using your rend and going back to healing.

"Why shouldn't I run cards to heal me or give me movement during shadow travel?"

Because, let me reiterate, shadow travel is a last ditch effort. You should not spam it. You get more consistent movement using Umbral Gait, which will persist if you don't Q before shadow travel. Your healing through other means is also better than healing through shadow travel. You should be using it to get away and reposition while your team refocuses, not to run away while your entire team dies, only to get chased down and stagger your death.

"But when you die won't you just lose all your stacks and have to get them all back?"

That's the neat part, you don't die. Rather, you die a lot less, and as mentioned previously it's really easy to get stacks. Over my last 12 games, I averaged 2.3 deaths per game with a K/D/A ratio of 5.4/1/5.1, while not missing out on healing. If I can do that running this game on an 8 year old trashy laptop, I know you can do better.