Zeke’s Harbinger

There's a new support item out! Let's figure out when you should build it.

There’s a new support item out! Let’s figure out when you should build it.

Zeke’s Harbinger
Cost: 2300
Build Path: Glacial Shroud + Amplifying Tome + Amplifying Tome + 480 Gold
Mana: 250
Armor: 35
Ability Power: 50
Cooldown Reduction: 10%
Unique Active: Bind to target ally. This will remove all other conduits on your target (60 second cooldown)
Unique Passive – Conduit: When within 100 units of each other, you and your ally generate charges, stacking until 100 are reached and lasting for 8 seconds. Attacking or using ability generates extra charges. At maximum charges, causing damage consumes all charges and increases you and your ally’s ability power by 20% and critical strike chance by 50% for 6 seconds.
Also, for reference, other support itemization options:
Locket of the Iron Solari
Ardent Censer
Mikhael’s Crucible
Banner of Command
These four items constitute the best other options for third item on a support (after support item/sightstone).
Realistically, this item will be completed by about the thirty minute mark, and in an average length game, it will probably be the last item the support builds. Objectively, it’s a great item. If I was an ADC, I would always want my support to build this item. It’s vaguely reminiscent of the old Sword of the Divine (Remember that item?), only better because it has a significantly longer duration. However, the biggest problem with Zeke’s Harbinger is that the other options are simply too good. 
At this point in the game’s life, the support is probably always building Locket of the Iron Solari, unless the jungle is feeling generous or there is negligible AP damage on the enemy team. Even if the Locket is not a priority, Mikhael’s is way too strong with its empowered cleanse; while supports with AoE shields/heals (Karma/Alistar) will always want to itemize Ardent Censer. With this in mind, when considering Zeke’s Harbinger, one must take a step back and consider what it brings that other other items do not. 
Banner of command – siege utility
Locket of the Iron Solari – teamwide defense
Mikhael’s Crucible – individual defense
Ardent Censer – teamwide offense
Zeke’s Harbinger – individual offense
So, with that in mind, Zeke’s is an item you’re going to want to pick up if you:
A. Don’t have to itemize Locket and/or
B. Have a carry who relies heavily on crit and is stupidly fed,
The carries who first come to mind when I think of crit are:
Realistically, almost any support can build this if the situation calls for it (I can’t tell you how many times I’ve built Mikhael’s on Leona even though she doesn’t really have mana issues). Obviously this item generally synergizes better with mage supports, but I would pick it up based on what the landscape of the game is looking like/who my carry is playing/how fed they are. Take a gander at your team’s win conditions before choosing your third support item and pick accordingly. And if the enemy support has a Zeke’s Herald to use on their fed Twitch, be very very afraid.