The mechanic
The Obsidian Golem is the classic splitter. It arrives as a single heavily armored construct that grinds slowly up the path and does heavy damage to your base if it reaches it.
When you kill the big golem it does not simply die: it splits into 3 mid-sized golems. Kill each of those and it splits again into 3 small golems apiece. One giant therefore becomes a full generation of 1, then 3, then 9 bodies, and every one of them is still walking toward your base.
The top tier is armored and resists crowd control more than the smaller pieces, so slows and stuns bite less on the big form and land more cleanly on the splits.
How teams beat it
Do not pop the big golem until your towers are ready to handle the flood of splits it releases, because clearing it too early near your base lets the small golems leak. Bring splash and area damage to mop up the 9 small golems efficiently.
Like every boss, it fights inside the showdown rules: kill it and it respawns about twenty percent stronger on the rival's lane, let it leak and it crosses unescalated. The full showdown rules are in How a match works.
