...

In my opinion it's the lifesteal that is holding the item back. You get lifesteal and a counter to sustain in one item. Give it an upgrade and it would be too obvious of a pick for AD champions. I personally like:

Executioner's calling
Long Sword (415g)
Brawler's Gloves (400g)
535 g
+15% critical strike chance
+15 attack damage
Unique: On-hit, you cause 4 damage per second for 8 seconds. Unique: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (20 second cooldown).

Effectively you make an item that makes it easier to deal with sustain because you decrease their healing and do more burst damage to them, rather than decreasing their sustain while increasing yours.

I'm less sure about a good upgrade path, but I like the idea of combining it with Brutalizer. Say:

(Name TBD)
Executioner's calling (1350)
Brutalizer (1337)
500g

+50 attack damage
18% crit chance
+20 armor penetration
+15% cooldown reduction
Unique: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (20 second cooldown).

You have an item that is generally good on an AD based character, and situationally strong vs high sustain teams. It's also rather attractive to AD casters. You now have a choice when building Brutalizer as well--do you go for ghostblade with it's GP5 component or do you go for the upgraded calling?

I like the above choice, making it an AD item.
Also like the idea of combining it with Brutalizer. But here's something out of left field, assuming Ueber's changes to calling itself:

Executioner's Regalia
Executioner's Calling (1350)
Cloak and Dagger (1450)
600gp = total: 3400

20 Attack Damage
35% critical strike chance
30% Attack Speed
+40 Tenacity
Unique: On-hit, you cause 4 damage per second for 8 seconds.
Active: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (20 second cooldown).
Special: When an effect that would be reduced by tenacity ends on your champion, all targets currently affected by either of Executioner's Regalia's unique effects gain an additional two seconds of duration on such effects.

Why? Because both EC and C+D are underrated sleeper hits, and I want a bloody end-tier tenacity item, and something that you can use to punish chain CC.

It needs to either build into something or get completely reworked.

Builds Into one of these 2..
Kessler's Static Spear
+40 Attack Damage
+20% Critical Chance
+20% Lifesteal
+10 Mana Regen.
UNIQUE: On-hit, you cause 15 magic damage per second for 8 seconds.
UNIQUE Active: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (40 second cooldown.)

Executioners Calling (1350g)
Pickaxe (975g)
Meki Pendant (390g)
Recipe: (425g)

TOTAL: 3140g

------------------------------------------------------------------------------

Kessler's Static Spear
+45 Attack Damage
+20% Critical Chance
+20% Lifesteal
+20 Armor Pen.
+15% Cooldown Reduction
UNIQUE: On-hit, you cause 10 magic damage per second for 8 seconds.
UNIQUE Active: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (40 second cooldown.)

Executioners Calling (1350g)
The Brutalizer (1337g)
Long Sword (415g)
Recipe: (625g)

TOTAL: 3630g

------------------------------------------------------------------------------

~Balance/Buff~
Executioner's Calling
+15 Attack Damage
+15% Critical Chance
+18% LifeSteal
Unique: On-hit, you cause 5 damage per second for 8 seconds.
UNIQUE Active: Inflicts target enemy champion with Grievous Wound, causing 50% reduced healing and regeneration for 8 seconds (30 second cooldown).

Brawler Glove (400g)
Vampiric Scepter (450g)
Long Sword (415g)
Recipe (350g)

TOTAL: 1615g

These are sort of nerfs to it's current strength's of the active....also I wouldn't call slapping 8 damage onto it's current form and doubling the CD a balance. Making something weak doesn't make it balanced, just weak. Right now, I think the item needs an overhaul, and to be added into a recipe for a new item, what that item is I'm not sure.

Ap grievous wounds > Ad grievous wounds

:'(

I'd like them to rework EC and ZF both into simpler weaker items and combine them to offer aura lifesteal, AS, and crit, along with double LS for the wearer and the active effect as well, they can just drop the passive damage, health and cooldown from the recipes.

20sec is far too short a cooldown and completely unfair on champs like mundo and swain.
40sec is shorter than most cooldowns and if you have ignite you've got a back up.

If you have Ignite, why would you buy this?

Don't forget about some of the unique utility that the damage/time brings. The damage/time does not hurt much, but regardless makes it so that:
-You get the assist even if the champion dies from another source 10-18 seconds later;
-The champion can not recall, activate traveling treads, or get some passives (Garen, Miss Fortune, Teemo) for 8 more seconds
-[Dominion] Cannot capture a turret for 8 seconds after disengaging
-Edit: delay the character from entering stealth

If the item gets reworked, I would really, really prefer that this effect remains similar since its so unique and interesting.