Found a Trace of Echoes in Diablo 4? Do not let it rot in your inventory
The Crab with the Knife makes Echoing Hatred worth it if you have the chance!

The season is coming to an end, and there is one particularly rare piece of endgame content you may want to check off your list: Echoing Hatred. If you have a Trace of Echoes sitting around, now is a great time to use it, because who knows if you will find another one in Season 15.
If you have never heard of it, Echoing Hatred was introduced with the Lord of Hatred expansion as an extremely rare, wave-based endgame activity. Blizzard even described it as a "hyper rare" event because getting inside requires finding a Trace of Echoes through normal loot, and those are really rare.
It is also frustrating because you can only hold one at a time, and if you are really unlucky, you can even go entire seasons without finding one. They did increase the drop rate, but the item remains rare enough.
How Echoing Hatred works
Once you find a Trace of Echoes, take it to Temis in Skovos and use it at the appropriate altar to enter Echoing Hatred. Typically, if you currently have a Trace of Echoes on you, you will have a priority quest that automatically activates and leads you there.

The activity is pretty simple. You have to beat increasingly difficult waves of demons, and the difficulty is based on Pit tiers. As you defeat demons inside the Echoing Hatred, you progress through different tiers, and each of these is equivalent to a Pit Tier. So it is basically some sort of giant Pit where you defeat all the waves and tier bosses in a single room up to your max. If you die, the run is over.
There are also four shrines inside the arena that you can activate:

Each can be activated once, so using them at the right moment can make a big difference during a longer run.
The Rewards Are the Real Reason to Run It
Echoing Hatred rewards so much gear, crafting materials, gold, and more. It will literally shower you with loot.

Treasure Goblins can also appear during the event, and killing them gives you additional Goblin's Plunder on top of the rewards waiting at the end of the run. In other words, if you manage to push far into the event, there is a lot of stuff waiting for you.
There are also a couple of special rewards worth knowing about. Completing Tier 60 or higher for the first time unlocks the amazing Crab with the Knife pet. Yes, Diablo 4 has a tiny crab holding a knife. And yes, you should probably get it.

There Is Also a Secret Reward
Echoing Hatred hides another reward behind a little secret puzzle. If you activate the shrines in the correct order at the corresponding tiers, you can unlock the Howling Mouth of Madness town portal cosmetic.
The required order is:
Protection: Tier 1-9
Artillery: Tier 10-14
Lethal: Tier 15-19
Channeling: Tier 20-24
This is particularly important if you are doing Echoing Hatred for the first time, since the Trace of Echoes is so rare that you probably do not want to accidentally waste your opportunity at the portal reward.
If you are playing in a group, be careful as well. Other players can activate the shrines too, potentially ruining the required order. But you only need one Trace to enter, so you can definitely do it with a full group.

You should also check the vendor at the end, because he often sells good gear that you can use for crafting.
In the end, Echoing Hatred is absolutely worth running if you have managed to find a Trace of Echoes. Even if you already have the portal and the pet, if you were lucky enough to find a Trace of Echoes, it is definitely worth doing before the season ends, just because of the amount of loot it can give you. It is also a good way to carry players that are maybe a bit lower level or are trying to finish some stuff before the end of the season, so if you have a key and you are alone, putting it up in the party finder can also be a good way to help beginner players get a new pet and portal.
