Deerclops is a boss enemy introduced to Terraria as part of the Don't Starve Together crossover. It appears in the Snow biome and behaves as a large, single-entity boss with multiple distinct attack patterns and a notable invulnerable state when players are far away. Many crossover items are dropped by Deerclops when defeated. The encounter is unique among non-event bosses for not despawning immediately after killing the player and for consisting of a single destructible entity (the other single-entity boss is the Empress of Light).
Deerclops uses four primary attacks. If close to the player it raises its arms and smashes the ground, summoning a wave of Ice Spikes that travel rapidly toward the player; every third consecutive wave is shorter-ranged, has a slightly longer windup, and spreads outwards from both directions. Ice Spikes apply the Frozen debuff for 0.33 seconds, which can interrupt grappling hooks. When too far from the player to use the Ice Spikes attack, Deerclops will "scoop" and hurl numerous chunks of Debris that travel in large vertical arcs and can pass through blocks; the Debris appearance depends on the blocks Deerclops stands on but does not change damage. Periodically Deerclops will roar and apply the Slow debuff to all players within 50 tiles for 12 seconds. If the player spends enough time above Deerclops it will roar and summon five Shadow Hands around the player; in Expert Mode Shadow Hands are summoned more frequently as Deerclops loses health and can behave differently (slow-seeking, fast-curving, or spinning). Shadow Hands predict player movement and pose a significant threat when players linger directly above the boss.
When the player is more than 30 tiles away, Deerclops enters an invulnerable, cloaked state: it becomes surrounded by a shadowy aura, emits black smoke particles, turns a dark purple while its eye turns magenta, and gains two reddish semi-transparent afterimages orbiting it. While cloaked it is completely immune to damage until a player approaches within 30 tiles, at which point it reverts to normal appearance and behavior.
Warmth Potions and a Hand Warmer are recommended to mitigate the cold environment and reduce risk from the Frozen debuff.
Constructing a platform of non-solid blocks about 6 tiles above ground with solid staircases on the sides can keep Deerclops high enough to prevent its Debris attack, simplifying the fight.
Mobility aids (Anklet of the Wind, Magiluminescence) combined with Swiftness Potions help counter the Slow debuff and improve survivability.
Avoid staying directly above Deerclops to prevent the dangerous Shadow Hands attack.
You can safely run between Deerclops's legs to evade the first two Ice Spike waves; do not remain there to avoid the third spreading wave.
Place your spawn point away from Deerclops's possible wandering area; the boss will remain in its spawning Snow biome if no players are nearby rather than immediately despawning, and it can linger near spawn—this is especially important for Mediumcore characters.
If all players are dead or none are nearby, Deerclops returns to the Snow biome it spawned in and will despawn after one in-game day (24 real-world minutes) or if a player leaves and re-enters the world.
Additional details: while Deerclops is alive, the Blizzard visual effect's opacity is reduced. Deerclops cannot normally pass through solid blocks but will rise through blocks quickly if the player is above and out of reach. Defeating Deerclops is not required for the Slayer of Worlds achievement. Deerclops's purple invulnerable form visually resembles its appearance in Don't Starve Together, and its name is a portmanteau referencing that source.
Slayer of Worlds achievement.