cool untitled sequence thingy
Ruby, 32 28 23 bytes



-&gt;n,k{k.step(0,-n).sum}


Try it online!

Explanation

Let's visualize the sum as the area of a triangle, for example with n=3 and k=10:

*
*
*
**
**
**
***
***
***
****


Then we sum by column instead of row: the first column is k, then k-n, k-2n and so on.