I'm not Phantaskippy, but I figured I'd contribute with a guide to one of the promo spirits.
Theme
Serpent is an immensely powerful spirit that's only just waking up in response to the invaders. Your presence tracks are very weak to start, but eventually branch out into something that no other spirit can match.
Unique Mechanics
Your Deep Slumber ability is a big liability, stopping you from placing more presence until you've used Absorb Essence a few times. When counting up presence track slots, remember to add one for your initial presence, which means you need to absorb twice before you can get the Earth element in the middle of your board. Also, Deep Slumber only cares about presence on the board, so if you lose some to blight, you can place more. This also means that you (and your teammates) shouldn't place your presence with Blazing Renewal.
Presence Track and Growth options
Your growth options are pretty solid. Nothing that's all that powerful, but you can be pretty flexible. You basically always want to place a presence, unless you have a very good reason for doing otherwise. Serpent's growth is slow enough, and you really don't need it to be any slower.
Your presence track is one of Serpent's defining aspects. You take a very long time to get online, and you'll be behind basically every other spirit until you hit the earth element and then explode in power. It's not all bad, though. You get a ton of elements which can make hitting thresholds very easy, you hit 2 card plays reasonably early, and you can get a reclaim one space by turn 3.
Starting Powers
Absorb Essence is vital to your continued success as Serpent. Try to use it on spirits who don't need too much presence on the board or have long range powers. Also be careful about disrupting someone's sacred site.
Elemental Aegis is a purely defensive power, and it only does one thing, but it's pretty good at that job, and combined with Serpent's 3 range presence placement, you can often show up and block a ravage. Early on, when card plays are limited, you'll want to be using powers with more impact, once you're at 4 plays it's a very cheap way to contribute at range.
Gift of the Primordial Deeps is more substantial early on when an extra minor power can represent a turn's worth of growth options and pushing a reclaim turn back, while Gift of Flowing Power is stronger later on when spirits have larger hands and an extra power played doesn't just mean they get forced to reclaim early.
Innates
The first effect of Serpent Wakes in Power is decent, and easy to get, but not really a game-changer.
The second effect is a lot stronger, but it's hard to get early on. You can do it with the Any element on the energy track and 2 card plays, but that requires two powers that both have Earth, Water, and Plant, which requires some luck. It gets a lot easier if another spirit took Elemental Boon, or you're playing with a Bringer who's willing to give you an extra element. You can fish for minor powers (there are enough that you have a decent shot at meeting the requirements), but in a lot of games you'll just have to wait until you get the Earth element (which allows you to use the Any element on Plant, so you'll just need to play Elemental Aegis or Absorb Essence, plus get any plant power).
Gaining a Major power is pretty great, and can be done with two of your starting powers (Elemental Aegis + Absorb Essence) plus any two plant powers (once you hit 4 card plays).
The first two abilities of Serpent Rouses in Anger are very easy to trigger, and you'll easily be moving or destroying a town on turn 2. Because of range 0 and the power being slow, both versions are best used to hit a freshly built town.
The last ability is only going to happen in the very late game considering all the things you need first (high energy gain, the right elements on your powers, and either 5 card plays or the second Any element).
A note on Solo Serpent
Serpent in a solo game breaks a lot of rules, doing things like getting extra elements from a single card, absorbing its own essence, and using the gifts to play more cards than its card play tracks should allow. Solo Serpent is a whole different beast and a lot of aspects of strategy need to be re-evaluated. Though do note that your own essence doesn't let you double up on Serpent Wakes in Power, since that specifies other spirits.
Play Style and Strategy
The devs have stated that Serpent can focus on either track early, gets forced to go balanced to get the earth element, then can specialize again.
What I'm going to suggest here is just one way to play Serpent, and it's an energy-and-majors strategy meant to get you active and carrying your own weight starting from an early turn. I haven't tried a plays-focused strategy for myself, but I imagine it'll be a more defensive, supportive strategy based on the fact that you'll be reclaiming and playing your gift powers more often.
Turn 1: Place presence from energy (try not to put it on a land that is about to build) and gain a minor power. (It doesn't really matter which, but if you can find one that has plant and earth, lean towards that one.) Play Gift of the Primoridal Deeps on any ally. Both plays-focused and majors-focused spirits can make good use of it, as well as any spirit who's one moon or earth short of an innate.
Turn 2: Place presence from energy again (put it into a land that's about to build), then gain a major power.
You want a damage or destroy power that ideally:
- Costs 6 or less (so you can support it for a while with your +4 power growth option and energy track, plus the 3 from Absorb essence)
- Doesn't require a sacred site, since it makes targeting awkward.
- Has thresholds with fire, water, moon, or earth. Low thresholds can potentially be met with the any element. If you can't meet thresholds, pick powers based on their base effects.
Your subsequent power choices will build around whatever power you picked here.
Play Absorb Essence, and pick earth for your any element. This lets you do 2 damage with Serpent Rouses in Anger, which removes a town. Alternatively, you can play Elemental Aegis to stop the ravages while still stopping the towns, but that slows your major power by a turn, which causes its own problems. If your teammates agree, I'd recommend sucking it up and taking the blight, especially if you took a power that can remove it later.
Turn 3: Place presence from Energy and reclaim Absorb Essence. If you can afford to (your power costs 5 or less), gain a minor power. Otherwise, you'll need to gain energy. (This is true even if your power costs 6 and you have 4 total power before growth, because you need to have the energy for next turn as well). Play your major power.
On turn 4 just reclaim and play your major. On turn 5, when you uncover your second card play, play Absorb Essence and any power you can afford to. This puts you in a good spot, ready to uncover the earth element two turns from now, and go into the lategame with a good hand, sculpted around a major power. You could even forget about thresholds and prioritize the 6 energy space, and sling two major powers in the same turn.
If you have a teammate like SRG, River, or Wildfire, you can get away with gaining a major and playing Absorb Essence on turn 1, then starting the major-reclaim train on turn 2. If your draw sucked, you can even gain another power card on turn 2 instead of getting energy, though that means you can't get away with a 6-energy power.