SMITE

Thanatos' third ability, Soul Reap ( if you use standard controls), is an area of effect silence. Since it says "As Thanatos, silence 15 enemies out of their abilities," you need to silence them WHILE they are doing the ability - meaning this only works on channeled abilities.

This does NOT have to be done in one game; it is cumulative.

Brief explanation of channeled abilities for new players:
Most abilities in Smite, you press the button, and the ability happens, and it's done. A channeled ability is an ability where when you activate it, your character continues to do an animation and carry out the ability. Ie, they take longer to complete, meaning you can be interrupted.

Here are some examples:

Anubis- Plague of Locusts. He sprays out pestilence for 3 seconds.
Ares- Searing Flesh. He spews flames for 4 seconds.
Bacchus- Belch. Bacchus burps for 2 seconds.
Bellona- Bludgeon. Bellona spins around, then dunks with her hammer.
Cabrakan- Tremors. Cabrakan stands still and punches the ground for up to 10 seconds.
Guan Yu- Taolu Assault, the ability where he spins his swords for 3 seconds.
Isis- Wing Gust. Isis flaps her wings 4 times.

Also note - many ultimate abilites (in fact, most) in Smite are channeled, such as Ra, Thanatos, Anhur, etc.), but since they are crowd control immune during their ultimates, silencing them will have no effect.

There are many more, but I wanted to keep the solution on the shorter side, and I thought these examples were easily recognizable. Please let me know in the comments if you have any questions, or if I can improve this solution. Good luck!

Play as Thanatos and use your 3 on enemies when they are in the middle of doing an ability, it will cancel their ability and silence them for a few seconds. Do this to 15 enemies, also this does not have to be done in one game