It is a good buy versus anyone who has ultimates or abilities that rely on health regeneration.
The active makes this item extremely effective against champions with healing abilities, as well as the use of health potions.
It is also effective against champions and item builds that rely on spell vamp and/or life steal.
Examples of champions/abilities/items that it counters include: Health Potion
Dr. Mundo's Sadism Fiddlesticks's Drain Soraka's Astral Blessing and Wish Swain's Ravenous Flock Warwick's Hungering Strike and Infinite Duress Sion during Cannibalism Tryndamere's Bloodlust Renekton's Cull
the Meek Volibear's Chosen of the Storm Vladimir's Transfusion
The Bloodthirster / Sanguine Blade Summoner Heal Source
So, if there is particularly abusive Swain, Warwick, or Sion then it can help stop them.
gaming.stackexchange.com/users/3015/Rapida
Executioner's Calling is indeed an interesting item.
I personally find it to be underused, but there are only a few characters that I feel can make use of it.
Gangplank is a good champion to use this on/against.
Being able to cast this on Gangplank as you pressure him will prevent his heal from being as effective.
Also it is generally useful against any champ that relies on sustain in lane.
Sadly I think it needs a buff, as there are very few characters that can really benefit from grabbing this.
As you've noted, also a good mundo counter if your team does not have ignite
/Trist/
Fizz/any other heal debuff
gaming.stackexchange.com/users/22123/zYn
This is one of my favourite items in the games.
It is similar to Wriggle's Lantern, in that it helps you out in-lane but is probably not something you will be keeping in late-game.
The stats on it are pretty good for the cost, I get this item on Gangplank if the opponent has any form of healing including pots.
He loves the crits and extra sustain, and the passive 4dmg a second on top of his grog soaked blade makes his Parrley harass even more powerful.
This item is super cheap, and is a real bargain on any champion that benefits from crits & lifesteal but also needs some way to shut down an opponent's sustain.
I should note that the damage over time will have you prioritised by towers, so you must be careful of this if you plan on diving.
It also helps stop people getting into stealth, porting back home, and it can keep Malphite's passive down perpetually.
gaming.stackexchange.com/users/31219/Hado
When your team has 1 or no ignites with them and they ahve a tryn/sion/mundo/yi on the other team.
Or pretty much any champion who has high lifesteal and is able to take a ton of damage and just heal up easily.
Its purpose is straight down to stop the healing affects and give your team the advantage to burst down the other enemy champion like fiddlesticks who heals so much from his w
gaming.stackexchange.com/users/22167/Kurt E
Lifesteal and spellvamp champs, against healers (heal spell, sona, janna ult to lesser degree, trydamere, nidalee), people using bloodthirster, warmogs, ect.
It is, in my opinion, not the best choice
but it could be useful against those.
Ezreal, graves, any ad carry with lifesteal for example can outheal in some 1v1 situations.
This would cripple them fairly effectively but the item is indeed very situational.
gaming.stackexchange.com/users/22102/Piskie
It's basically ignite: the item.
:)
Great against heal/regen-heavy champs.
Try it in an on-hit build! :D
gaming.stackexchange.com/users/22189/Mattarias
You could use it in case their carry is stacking lifesteal and that's somehow giving him too much survivability, you could use it against Mundo, Warwick, Sion, Fiddle, just about anyone with a heal, and it's very cost effective.
gaming.stackexchange.com/users/33739/Xmus Jaxon Flaxon-Waxon
I know  Executioner's Calling  reduces HP regeneration and healing, but who would this actually counter except for Dr. Mundo? I've seen this item built once even when I play Dr. Mundo pretty often.  Can someone shed some light on this item?
What could Executioner's Calling be used for?
gaming.stackexchange.com
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gaming.stackexchange.com/questions/58083
gaming.stackexchange.com/users/22177/Redeem
This item is super cheap. The active makes this item extremely effective against champions with healing abilities. It is a good buy against anyone who has ultimates. The executioner's calling is an items that can be used against champions with high sustain by life stealing.
The community explains what executioner's calling does.