I wrote this tactica a mere 6 months ago for Warseer, though it seems like much longer. The metagame has singe changed a bit with IG, but the tactic is still relevant and useful. The original article is currently sitting smack in the middle of a thread currently at 4,500 replies, so I've "rescued" it over here. What? I never said that every single article here would be NEW, did I?
What is Serpent of Fury? In short: the act of deploying a medium-range shooting unit from a Wave Serpent such that the tank blocks enemy capability for reprisal into the unit.
The name "Serpent of Fury" (SoF) is a derivative of the 4th edition Tau tactic "Fish of Fury". Tau Firewarriors normally come with a Rapid Fire Pulse Rifle. Getting maximal output from this type of weapon means engaging in the 1-12" spectrum. The trick to this? FWs are rather fragile, especially in Close Combat. Thus they need to limit or eliminate the ability for the enemy to hurt them. The solution? They can buy a Devilfish Skimmer Tank as a Dedicated Transport. The Devilfish would move 6-12", and stop about 6" away from the enemy. It was even still be free to shoot it's weapons if you took the right wargear. As the front and side armour is the same, the tank would also turn to present the longer side facing to the enemy. FWs on board would then disembark on the side opposite the target and fire "under" the tank, as Skimmers did not block Line of Sight. In the enemy turn, the Skimmer would be well protected from shooting and assault due to the Skimmers Moving Fast rule. The guys behind would be protected from assault due to the bulk of the Devilfish. All around this was a win-win tactic for the Tau player. The unit could function independently, hit hard, was highly mobile, difficult to destroy and generally able to compensate for most drawbacks. This was such a highly effective tactic that it became staple for most high-end competitive Tau armies.
Then along came 5th edition and it's changes to SMF, True Line of Sight, and other changes. Suddenly Tau commanders were scrambling to put taller stems on their tanks, finding the Devilfish isn't nearly as durable, lightly armoured enemies were getting a cover save and the Tau were generally left lamenting that their trick pony had been hobbled. It is still feasible for them, but is iffy for a unit operating on its own. The tactic has been reduced from a staple, to something that either requires multiple units for area saturation or as an act of desperation.
How does all this apply to Eldar? Simply put, Eldar and Tau have a number of similarities. It's like we've been guiding the development of that lesser race for millenia.. Serpent of Fury existed in 4th edition play, but mostly as part of a mechanized theme or gimmick. The amount of damage it did was usually not worth the cost. Competitive play tended to favor more effecient tactics. This changed slightly with the "new" Eldar Codex when Dire Avengers shifted over to Troops for everyone and their range was extended to 18". This allowed them to fill a compulsory slot and also maintain a bit more distance from the enemy. SoF still wasn't a mainline item, but it was at least something to do with your required Troops selections when you had a mechanized army. But the clincher was 5th edition: Troops only as default Scoring. It was no longer enough to just pay token homage with 2x min sized Troops sitting off to the side... your Troops suddenly needed to DO something.
Excepting a weird mission or special rule, having surviving Troops is the only way you can get better than a tie in 2/3s of the current missions. This means that your Troops are going to be a primary target for any opponent interested in winning. To protect them, your Troops either need to be plentiful (Orks, IG, Nids or other hordes), tough as nails (Wraithguard or Plague Marines), and/or otherwise protected (bunkered in cover, hidden in Reserves, embarked in a tank, etc).
Side Note: There's also the Multiple Small Unit option, where you take lots of minimal sized Troops. However, this causes Kill Points to rocket in Annihilation missions and is not a recommended strategy.
For Eldar, hordes are unwieldy, rather static, suffer in CC, and anti-horde is becoming common thanks to the Orks. A Wraithguard core is solid but also slow and expensive in both points and cash. It's also a "jealous" choice in that lists don't tend to just include them, but instead have to be built around them. Relying on the batlefield to have enough cover to protect your guys is also be an iffy prospect. This is doubly true for Troops as they're there to claim objectives, yet this often means moving out of their original deployment position. There's also the idea of hiding behind a building or in a tank all game and heading out to claim objecties last-minute, but this takes timing, patience, a little luck, and the rest of your force picking up the slack.
With history and the general concept out of the way, let's take a look at some common questions:
1. Why a Wave Serpent and not a Falcon?
a. SMF is the same for each and even then only matters if you move over 12".
b. A Wave Serpent can carry twice as many models.
c. The Serpent Energy Field. It works no matter what speed you move. While the Falcon does have the protection of a Holofield, the Serpent Energy Field actually protects similar against many threats. But it works better when moving Fast and/or when shot by certain types of weapons such as meltas and Demolishers. This makes them superior for closing on the enemy.
d. Wave Serpent weaponry tends to be more complimentary of close-range engagement.
2. Why Dire Avengers?
a. 18" range. Thanks to this they can get out to the side of the tank and fire cleanly into their chosen target. There's not any issues with weird LOS from their heads and trying to shoot under the tank. You don't have to worry about modifying your tank to be up higher either. Actually, a short stem is preferable because the tank will better Screen or even completely block enemy LOS to the DAs from other angles. 12" range or Flamer Template units such as Fire Dragons, Guardians, Wraithguard and Warlock Councils CAN be used in this manner. However, you have to be much more certain about crippling or destroying your target in order to escape retribution and/or getting charged by survivors.
b. Bladestorm allows you to maximize the fire output on a given round. This fits well with the SoF often spending their next turn repositioning to fire at a new target.
c. DAs tend to rate fairly low as targets when there's other big threats like Fire Dragons around. This means they're often able to perform this trick multiple times.
Now a few things to note with SoF tactics:
1. It CAN be done by a solo unit. However, a full unit of DAs and a 2xASC Exarch Bladestorming average out to only 3.7 MEQ kills. Against anything tougher than Tau or IG, I REALLY suggest target saturation in the form of two or more units moving in tandem. This has an added benefit of allowing defensive formations of multiple tanks.
2. Doom is a huge benefit to this tactic, especially against T4+ opponents. 2 units and a Doom have a rather good chance of dropping even large terminator squads, Monstrous Creatures and other nasties.
3. A Serpent carrying a close-combat unit can nicely finish out a formation. This naturally places them and their tank in good position to defend the DAs or carry the assault to the enemy. This role can also be performed by a Serpent toting close range or Template units looking to get closer and maximize kills.
4. Your DA will naturally be a little packed in when the get out to fire. Beware enemies with lots of templates, especially Large Blasts such as Whirlwinds. I suggest neutralizing or blocking LOS to these before you get out of your tanks. Deep Striking Heavy Flamers and similar can also be quite annoying.
5. The distance game only works against enemies that only move and Assault 12". Be careful of jumping out too close to an enemy with Fleet or Waagh!.
6. If you're confident in your tactics/positioning, hold off on Bladestorming right away. Put in a turn of normal fire, then Bladestorm and possibly assault in the next.
7. If the squad's been chewed up and is near or below half, mount them back up in their tank and scoot for safety. You're probably into turn 4 or so at this point anyways and those guys are typically more important grabbing objectives than trying to slug it out.
The tactics for a single unit are pretty easy to grasp. Multiple-unit tactics can be a little more tricky, so I've provided some pictures for you. In the examples below, the Iyanden (Yellow) Wave Serpents are carrying the DAs. The Saim-Hann (Red) is moving Fast and likely carrying Fire Dragons or Banshees. The first picture is starting locations and the second after moving and deploying. The image lines moved when I saved them, but I'm sure you can get the idea.
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Example 1 (Left): Isolation of a target and multiple overlapping Screens. The squad on the left is the target in this movement. Both squads and their Serpents have decent to clear lines of fire into the target. Screening of the target doesn't matter as they'd get a 3+ save anyways. Every enemy except the target has their LOS Screened or even blocked by intervening terrain and tanks. Only the Fire Dragon tank is not nicely Screened and it is protected thanks to SMF. This is true not only of the guys immediately nearby, but the entire rest of the table.
Example 2 (Center): Capstoning up the center. Here the two squads deploy in between the two tanks with the third forming a protective capstone. The target unit would be the guys just inside the woods. Note that the front squad is over 50% protected from the Predator by their tank and the rest of the enemy by the red tank. This is the best option for attacking a central point and/or an opponent that has turtled.
Example 3 (Right): Push back. This is an example of combining the SoF with cover to create a position that is resistant to being assaulted. Likely the front squad of Marines is the target. However, the Daemon Prince or Raptors could just as easilly be targetted across the nose of the Red Serpent. This is your best bet for drawing off or countering enemy assaulters with an 18" range. Your DAs are protected by the tanks and that enemy models can't approach withing 1" of them until assaulting. Then being behind/in the woods means a Difficult Terrain test. The 4.5" is a calculated risk, but one many opponents won't take. The downside of this is that the unit is open to ranged fire from the inside of the board; it would work more efficiently reversed towards the board edge.
I hope that this has been informative. I'll be expanding on some of the ideas touched on above, especially those of mechanized assaults, Capstoning and others. Cheers!