You can summon the zombie boss with the zombie boss spawn egg.
This is made at the altar. It is planned to add more bosses which the
more often you defeat them and the boss's loot scales with these levels.
That means that they drop equipment which also have level and so you
create an "infinite" grinding. Of course, the bosses' lives and strengths
also scale with the level. I have also developed a system to add completely
own monsters with own textures, but they don't look so good
that I will release this feature soon. However, all this is still a dream of the
future because I also work other features such as plants and want to revise
the blast furnace system so that it is suitable for multiplayer.