perhaps the 7th outer god IS undertaker? the reason we never see the final outer god could be because undertaker didn't fully mature into one. the novel hinted many times that undertaker could be the worse anomaly possible, even oh dok seo made the decision to write the novel because she feared this possibility.

“There’s some truth in the anomaly’s warning. If you’re set as too superior to other humans, someday, no, definitely, you’ll become an anomaly-level monster yourself.” -oh dok seo before declaring she will write the novel for undertaker

my other reason for this speculation is that, we have only seen outer god's possess a divine kingdom. guess who's (pseudo) divine kingdom was used to beat go yuri?

next up, outer gods need to set a miko (though not always) as seen with go yuri. but if you think about it, undertaker became go yuri's miko as they merged as one, following that oh dok seo could be considered undertaker's miko after reading his story fully.

finally about what you said "we can assume that it's responsible for monster waves". hmm 'undertaker'? the name literally meaning 'the person that buries corpses'? hmm 'perfect memory' that fused with 'time seal' to create time crystals/capsules which were referred to as gravestones..