One build that has jumped out as me, which may not be completely obvious on first reading of the Codex, is something that I like to call the "Commando" Allarus Shield-Captain:
<SHADOWKEEPERS> Shield Host -- (-1A for enemy in Engagement range and get to re-roll all to Wound rolls against characters)
-Shield-Captain in Allarus Terminator Armor (with Guardian Spear, Grenade Launcher)
-Relic: "Gatekeeper" Relic (replaces Guardian Spear -- same in melee, in shooting it is 24" Rapid Fire 2 S5 AP-2 Dmg2, automatically hits the target)
-WL Trait: SHADOWKEEPER unique trait that ignores Invuls for all attacks against Characters
So, how does this model work?
Well, he can deep strike for free (since he is an Allarus Terminator), so if your opponent leaves even a single 40mm slot in his backfield, deep strike the Shield-Captain into it. With the Salvus Ka'Tah activated, that means at 12" his Rapid Fire2 Relic shoots twice, resulting in 8 x S5 AP-2 Dmg2 shots that automatically hit.
Now, you have two options:
1) "Bully" Mode -- If your opponent has left any weak units hanging out in the back to hold objectives, do actions, screen characters, etc., then hit them with both the 8 "Gatekeeper" relic shots and also D3 shots from the Grenade Launcher (S5 AP-3 Dmg1) -- against small squads of T4/T3 units, you have a very solid chance of killing them in one volley, with the possibility of charging them (lucky 9" roll of course) if they do survive.
2) "Assassin" Mode -- If your opponent poorly positioned his characters and left one nearest to where you deep struck the Allarus Shield-Captain, then things get extra spicy. Hit the exposed character with the 8 auto hit shots from "Gatekeeper" and the D3 shots from the Grenade Launcher. However, thanks to the Shield-Host tactic, you get to re-roll all wounds against characters (even shooting attacks!), so those S5 attacks get full re-rolls. You also get to ignore Invul saves against Characters (from the WL Trait above), so characters with a 3+/4++ are brought down to a 5+ save for the AP-2 "Gatekeeper" and a 6+ save for the AP-3 Grenade Launcher. Against many of characters in 40K, this should have a very strong chance of "one-shoting" them, thus removing that lynchpin from your opponent's army.
Alright, so what happens after this Deep Strike surprise attack? Well, this "Commando" Shield-Captain is not done. First of all, you can keep him in the Salvus Ka'Tah all game with a 1 CP Strat that lets one unit take a different Stance than the rest of the army -- this means that "Gatekeeper" is putting out 8 x S5 AP-2 Dmg2 auto hitting shots every single turn, so you can continue to just "torrent" down small units and characters for the remainder of the battle. Additionally, your Shield-Captain is still quite deadly in melee (especially against characters), with -1 attacks to any enemy units in Engagement Range of him, full re-rolls to Wound and Ignoring Invuls against characters, and 6 x S7 AP-3 Dmg2 attacks from "Gatekeeper" in close combat.
In terms of surviving enemy counter-attacks the turn after he arrives, well if you drop him into cover he has a 1+/4++ save with T5 8W against shooting attacks. You can also use Stratagems to prevent enemy re-rolls, make their attacks -1S, make them only wound him on a 4+, and/or reduce their charge distances by D6", so lots of ways to keep him alive while he is separated from the rest of your force.
Finally, say he does kill all the enemies around him... what do you do for the rest of the fight? Well, aside from grabbing Objectives (he counts as two Objective Secured models, remember), you can also use the new Strat to teleport over to a friendly Vexilla and reinforce the rest of your army with his re-roll 1s to Hit aura and strong shooting/close combat abilities.
What's the best part of all this? In an army of elite units where points are tight and every model you can squeeze in your list counts, he only costs 125 points and a Relic/Warlord Trait, meaning you have about 94% of your points remaining to still buy plenty of other Custodes units and, if you do lose him on his "slightly-suicidal-deep-behind-enemy-lines-commando-mission," you haven't crippled your entire battle plan.
Edited by L30n1d4s, 10 January 2022 - 01:18 PM.