Where selecting Orks detachments is a chaotic free-for-all and choosing Space Marines detachments is more regimented and orderly, the Adeptus Custodes sit somewhere between the two. Only one group of their detachments has a Unique keyword, while the rest can be chosen freely to boost units like Dreadnoughts, Terminators, or Sisters of Silence.
Six of the 13 detachments available in the Codex have the Unique: Shield Host tag, five of which are themed after one of the main Shield Hosts laid out in the lore section.* The Aquilan Shield, for instance, plays a more defensive game, focused on occupying objectives and whittling down the enemy as they’re forced to come to you, while the aggressive Dread Host charges forwards as fast as possible to brutalise their foes in close combat.
One thing you’ll notice – probably because we mentioned it on Friday – is that the Shield Host detachments each have a favoured ka’tah in addition to their detachment rule. These give additional effects when the corresponding ka’tah is activated, so if you activate Dacatarai for your units while using the Dread Host detachment, those units will re-roll 1s to hit against non-Monster/Vehicle units and reduce the pile-in and consolidation moves for engaged enemies by 2” – giving them a serious advantage over larger enemy units that now can’t get all of their warriors into engagement range.
Don’t worry about scuppering your tactical options, though, as having a favoured ka’tah doesn’t prevent you from using any others. It’s just a bonus.