I got a reward consisting of Soulstone trunk. When opening it, I saw this:
However, there was nothing new anywhere. I couldn't find how to use it.
Where are those soulstones and how can I use them?
Tap on your Heroes Altar, then tap Hire Heroes. Scroll to the right until you see the "Hire with Soulstones" option, then tap on that. That will open the dialog where you can redeem 200 Soulstones for the equivalent hero:
Once you're there scroll to the hero you want and if you have enough Soulstones you can hit Exchange to get a copy of said hero.
Note: this answer was written for game version 1.3.16 - IGG may move this dialog in future versions.