Things you need to consider: Line of Sight Usually, nothing will attack you, if they dont have line of sight, exceptions are walls as they are "enemies" pretty much.

Therefore, when you dig a hole, make sure everything is covered, it may work with little lines in the open, however you may still run the risk Yagluth somehow sees you and starts attacking. I recommend using the little square tiles to cover any small open spaces.

Yagluth also does not seem to like Fireplaces, if there is any fireplace around, its 99% likely that Yagluth starts attacking it and then (for some reason) also you. Granted, i did not try to get a fireplace down into the hole.

If you are out of reach, use a hoe to heighten yourself. Beware to not get too high as your head my clip outside, or your mace may also hit the ceiling and shares damage between yagluth and the ceiling.

If you can not hit Yagluth, may need to walk into any edges of your hole to force Yagluth to Walk around

This is the old HHc Seed, on this spawn you need to raise the ground level a good bunch so Yagluth may crawl down the summoning altar, if he spawns on the altar.

You may also need to bring yourself a forge down there to repair your mace. Since Yagluth got buffed, a lv1 Iron Mace took about 8 repairs If you are playing on pre-buff-patch, I believe, MDFV said you only need a lv2 Iron mace (no repairs needed)