This is an example of what the Enemy Spawner feature does. With items you can add modifiers like "+5 additional enemies" or "+15% Enemy Health" or "+25% EXP" to the Enemy Spawner.
The Enemy spawner has 4 tiers. Using an item that alters the Enemy Spawn's modifiers will add a tier. So starting from Tier 0, you can use 3 Enemy Spawner items to hit Tier 4, at that point no more Enemy Spawner items can be used until beating that enemy wave.
Here are some item ideas:
"Wrath Shard"
+25% EXP
+25% Enemy Damage
"Beserker Medal"
+35% EXP
+15% Enemy Damage
+15% Enemy Attack Speed
+15% enemy Move Speed
"Gold Stone"
+15% EXP
+25% Loot
