Demon Lord

Demon Lord is a boss that comes on 22 waves, from wave 18 to wave 119 (Acts I–IV).

Waves: 18, 24, 29–30, 34–35, 39–41, 44–45, 50, 57, 59, 80, 84, 89, 96, 101, 105, 114, 119
Element: Fire
Movement: Walks, 1.2 tiles/s
Armour: None
If it gets through: 3 lives

How to beat it

It is a fire creep: water towers deal it 1.5× (Chill, Frost Tower, Blizzard, Frost Bolt, Glue, Permafrost, Frostbog and Ice Shot and 13 more). Nature damage is weak against it (×0.75 on a boss).
A boss that reaches the goal costs 3 lives, so give it the longest walk past your strongest single-target towers.

Demon Lord on Normal, solo (the first wave of each act it comes in)
Wave	Act	Per lane	HP	Regen / s	Kill gold
18	Act I	1	5,794	–	45
34	Act II	3	9,260	–	31
80	Act III	3	21,604	–	16
96	Act IV	3	24,493	–	51

HP grows with the wave and the difficulty; in co-op every lane gets its own. Numbers from the game's data when this page was built.

The Last Maul

Co-op tower defence for up to 8 players. Free to play, nothing to install.