I stumbled upon two quotes from the Wiki.
"Liandry's Torment is probably best for ability power champions that can deal percent health magic damage like"
"Blade of the Ruined King is a great item choice for practically any autoattacking champion that does percent health damage like"
This is incorrect. Not that it cannot be true for Liandri's or BoRK to be good on one such champion, but that is never on merit of those two items dealing percent health damage as well as that champion having percent health damage. These two stats do not have increasing returns, they do not synergize with themselves. If anything it opens you more up to counterbuilding by focusing on resistances and offensive/utility items. 6 percent plus 4 percent is 10 percent. It's not like CDR or Penetration, which have increasing returns. It's like AD, ASPD, crit, AP. The first 10 points does exactly the same as the last 10 points.
Remember back in season 1 when everyone was really bad, building tons of HP on Cho'gath because "he already has a lot of HP"? I do. Maybe my friends were bad. Maybe I was bad and in the bronze of normals. But it annoyed me to no end. But I see some of the same a lot today.
It's not that having a lot of something makes more of that stat worth less - it just makes other stats that interact with it worth more.
There are however stats that synergize with themselves. That have increasing returns. One is Penetration. Flat penetration and % penetration works well together, flat penetration and flat reduction works good together as well. Penetration builds are particularly good on champions with great base damages and on champions that generally go for low-MR/Armor targets.
Another is CDR. Up to the cap of 40% naturally.
|y=percent more casts; x=CDR|
Then there's two other stats with some technicalities.
One is movement speed, which has 3 different stats that scale multiplicatively with each other. Flat movement speed, percentage movement speed, and the other percentage movement speed from certain abilities. Not all movement speed increases from abilities fall in the "other" group though.
However, movement speed is affected by two softcaps - 20% reduction after 420 ms and 50% reduction after 490. So movement speed increases act a bit odd, gaining efficiency until 420ms, then losing a bit before regaining even more efficiency, then losing a lot at 490 before gaining more and more and more efficiency. Reference the vids of Hecarims and Rammuses that go halfway across the map in a few seconds.
|Y=Post caps, X=pre caps|
The other technicality is healing, through any means. Net change in health is damage taken subtracted by healing taken. Once healing taken rises above damage taken, you're immortal. The reason why I state this as a technical but not practical self synergy is because of burst damage and, in case of lifesteal and spellcasts, crowd control. It is, however, still a phenomenon that can be abused in some situation. Particularly low CC and low damage scenarios, like one-on-ones and small skirmishes. (And why Aatrox is busted in 3v3. Aatrox Spreadsheet)