Added conditional based on existing fire damage conditional that will apply fire damage to a

mob standing on top of a magma node that belongs to the "fire" group. The magma node
has also been added to the fire group, making it so that standing on the node will
do damage to the mob.

In official Minecraft, magma blocks just do fire damage without the fire animation.
Source: https://minecraft.fandom.com/wiki/Magma_Block

mods/ENTITIES/mcl_mobs/physics.lua

@@ -644,6 +644,7 @@ function mob_class:do_env_damage()
 	end
 
 	local nodef = minetest.registered_nodes[self.standing_in]
+	local nodef2 = minetest.registered_nodes[self.standing_on]
 
 	-- rain
 	if self.rain_damage > 0 then
@@ -675,7 +676,19 @@ function mob_class:do_env_damage()
 				return true
 			end
 		end
+	-- magma damage
+	elseif self.fire_damage > 0
+	and (nodef2.groups.fire) then
 
+		if self.fire_damage ~= 0 then
+
+			self.health = self.health - self.fire_damage
+
+			if self:check_for_death("fire", {type = "environment",
+					pos = pos, node = self.standing_in}) then
+				return true
+			end
+		end	
 	-- lava damage
 	elseif self.lava_damage > 0
 	and (nodef.groups.lava) then

mods/ITEMS/mcl_nether/init.lua

@@ -130,7 +130,7 @@ minetest.register_node("mcl_nether:magma", {
 	is_ground_content = true,
 	light_source = 3,
 	sunlight_propagates = false,
-	groups = {pickaxey=1, building_block=1, material_stone=1},
+	groups = {pickaxey=1, building_block=1, material_stone=1, fire=1},
 	sounds = mcl_sounds.node_sound_stone_defaults(),
 	-- From walkover mod
 	on_walk_over = function(loc, nodeiamon, player)