Serigrass shards are way too expensive. They are 20 grandmother tokens each.
There are 3 weapons that require them: sepulcrum, trumna and quassus
I've been trying to get a shard for about 4 days, getting father and otak tokens constantly and have 9 grandmother tokens. It's especially annoying given how randomized it is and how you have to wait for offers to reset.
I'd say the quassus is the one of the most annoying weapons in the game to get. The bp is easy to get but then it required 5 scintillant and a serigrass shard.
At this rate maybe in about a month I'll have those 3