Diablo 4 has an easy-to-miss merchant selling Runes and Grand Gems
A simple War Plan trick can add a valuable shopping opportunity to your Undercity runs.
In Diablo 4 Season 15, it looks like everyone suddenly needs a ton of Runes, especially when you are trying to get rare ones such as 
Ber in order to craft 
Enigma. And if you are in this scenario, you are probably spending a lot of time inside the Kurast Undercity running the 
Greater Tribute of Harmony. But there is one War Plan node you might be overlooking. With the right setup, you can get a Cabochon Merchant to appear at the end of your runs. And even if his inventory is completely random, he can sell some pretty valuable items, including Ber and Grand Gems.
He is certainly not cheap, but if you are already farming Runes or working on your Gems, it is worth checking what he has to offer.
How to Get the Cabochon Merchant
To give yourself a chance of finding the Cabochon Merchant, you will need to select the appropriate nodes in your War Plan. Focus on the middle row and take the left path toward Portal Prankster.
Basically, you want to first unlock the left line until you reach the Portal Pranksters that can appear during your Kurast runs, to increase your chances of encountering one. And then take the middle path up to the Bauble Day node that guarantees a Cabochon Merchant if you killed a Portal Prankster during your run. The merchant only appears after you kill the District Boss.
Keep in mind that the Pranksters are not guaranteed. You need to encounter and kill a Portal Prankster first, and only then will the merchant show up. So you might complete runs where you will not see him at all. But when he is there, it is always worth taking a look at his inventory before leaving.
He Sells Runes and Gems
The Cabochon Merchant has a random inventory, and sometimes he has some genuinely useful items for sale. He is specialised in Gems and Runes. Most of the time he does sell only worthless low-level Gems and Runes that you do not need, but here and there you might really get some good stuff.
For example, some people found Ber runes or 
Grand Sapphire, or even Legendary Horadric Seals, and Season 15 even added a chance for Splinters to appear. However, keep in mind that he is not cheap at all. Ber Runes cost around 50 million gold each, and Grand Gems cost around 500 million gold each, for example. Those are some pretty hefty prices, especially for the Gems. But if you have been farming for a specific Rune or trying to upgrade your Gems, being able to buy exactly what you need can save you quite a bit of time.
Combine the Merchant With Your Regular Rune Farming
The goal is not at all to do Kurast just for a chance to get the Merchant. If you are already running Kurast Undercity for Runes, you can combine your usual Greater Tribute of Harmony with the Portal Prankster and Cabochon War Plan setup. This way you will still get your normal Undercity rewards, but you will also have an additional chance to find a merchant selling something useful at the end of your run. And if you are working toward something like Enigma, finding a Ber for sale could save you a lot of time. The same goes for Grand Gems. If you are trying to finish upgrading your Gems and have plenty of gold sitting around, buying one directly might be better than spending another few hours farming materials.
The Cabochon Merchant probably is not worth farming on its own, but it is a nice bonus chance to get the Gems or Runes you need in exchange for a bit of gold... or, well... half your gold, more likely...