To obtain Ominous Masks, you need to find and kill Hilichurls over Level 60+. Upon eliminating these standard enemies, they have a chance to drop Ominous Masks. Anything above Lvl 60+ also has a chance to drop Damaged Masks and Stained Masks.
To get Hilichurls to become Lvl 60+, you need your World Level to become World Level 5 or higher. The most efficient WL to farm is WL 6, as enemies are between Level 64 and 84. This will give you the most consistent enemy Lvl past 60+. Anything below you will find them, but not as often.
To get to WL 6, you need to have completed the following.
- Be on World Level 5
- Be Adventure Rank 45
- Clear Ascension Quest 3
You can get these from any of the Churl Family, such as Samachurls, Mitachurls, and Lawachurls, but not as often. This means killing bog-standard Hilichurls is your best bet, as they are the fastest and easiest to kill.
The quickest way to do this is to use the Waypoints, kill everything in sight, and then move on. It might take you a while to amass the numbers you need.
The two images below are where you can find Hilichurls in both Mondstadt and Liyue. Just click on them to enlarge them.
Thanks to Genshin Impact Interactive Map!
- Type: Common Ascension Material
Using an Alchemy Table, you can turn lower tier materials into higher tier materials.
Ominous Mask is used for the following Character Ascensions.
Each Common Ascension Material has three tiers. These are the tiers below Ominous Mask.