The second set of new rules is the Castellans of the Imperium. This detachment is much better than the first, though it is not featured for the Black Templars specifically. However, there are quite a few units that the formation has access to that the specifically BT-oriented. This list requires us to use at least two different Factions in the detachment, meaning Black Templars and one other. It also forces us to have 2 HQ units and 4 troops minimum. Other than that, we can take 6 of the remaining slots and/or 3 Lords of War. So you really can take whatever you want. The special rules are also much better than Wrathful Crusade. Remember our old Emperor's Champion Vows? Accept Any Challenge, No Matter the Odds? Re-rolling failed hits in close combat? Yep, we get it back with this formation. Zealous Defenders gives Hatred to everyone in the detachment. If you happen to have a lot of points to spend, you can max out the formation and gain Zealot instead of Hatred. Great, if you are playing a billion points. You'll usually just be rocking Hatred though. In addition, Flock to the Front Line allows you to roll a D6 if one of your troops units is completely destroyed. If you get 5+, you basically get a brand new unit (though in Ongoing Reserves) to bring back onto the board. Does this mean you can get respawning dedicated transports? RAW says yes. So get ready to possibly respawn Land Raider Crusaders.
With that being said, do I feel this detachment screams competitive? Possibly. But it does take a certain list to make it effective. First thing to note is that we have a HQ tax. Possible to overcome? Yes. Will it add to the list? Difficult to say. However, when we take Allied Detachments...we have a HQ tax, so I'm used to that. The great thing about this detachment for myself is that most of my lists already fulfill the basic requirements, just giving me extra rules. We cannot rely on the FttFL special rule, however, as its only a 33% chance to respawn the unit. What it does do, is give us a buffer if we're using MSU lists. When you have 6 units, we're going to get 2 units back on average if they die. This will give us a little bit of play and some additional tactical options in a game. I personally feel that aggressive lists, despite gaining Hatred, will not get the greatest use of this list. Why? Because our troops are not designed to be aggressive. Troops, nearly by definition, are objective holders. They hold ground, not take ground and we have to take a minimum of 4 troops.
When I see this detachment, I see lists like my Defensive Drop Pod list that will gain the most benefit from it. Hatred is definitely a huge boon to our lists, but because we have to take 4 troops in the list, we lose some flexibility with our points. Being able to utilize the Hatred to maximum army-wide benefit will be difficult; Thus, one option is to make sure that we have an unit or two that is specifically devoted towards close combat. Otherwise, we want to utilize the MSU approach to give it to all. Will these units be scary close combat monsters? No, but they are dangerous in specific situations. And always a close combat deterrent. However, building around the Hatred, I think is a mistake because there are other formations out there that have better special rules for killing your opponent's units.
To summarize: Wrathful Crusade, while thematic...does not have a place in competitive Warhammer 40k. Castellans of the Imperium, on the other hand, potentially is. However, do not rely on either Hatred or Flock to the Front Line to win the day. These special rules should be considered buffs to your list, not something to build around. It is an upgrade to the Combined Arms Detachment (though lacks Objective Secured) and need to be considered as such.
Now get back to Cadia!