Daddy Long Legs is a giant, spider-like creature with grey colors and six eyes; two larger while four smaller. His mouth sports with few ugly-looking teeth, a lolling tongue, and a lack of jaw. From the top of his head are four longer limbs made with bones and red-colored flesh.
He hovers in the middle of the room much like Mom's Heart. He has 4 different attacks:
Stomping all four of his feet around the room.
Retracting into the air and stomping one foot at a time.
Spawning 2 spiders by coughing them up, much like Widow.
Slamming his own head into the ground to stomp you, firing blood tears in all directions.
During his first attack, zoning doesn't matter — he'll stomp anywhere on the ground to catch Isaac while you're running. Look out for the shadows of his feet and avoid them at all costs. During this attack, Isaac can only damage Daddy Long Legs directly when he is "lowered" to the ground. Otherwise, Isaac will have to damage him by shooting his feet, which are rather thin twigs of bone.
During his second attack, running matters a lot. He'll follow Isaac much like Mom's foot, but at a more constant pace. His foot comes down quickly, but doesn't have the best range. Simply run a circle track around the room to easily avoid this attack. If Isaac don't have enough speed, he might want to try some other tactics like a bit of a wiggle line.
There is no strategy required for his final attack, as it simply happens once at the end of the second attack. Simply run at all costs until he slams his head down, in which he will get back up and stay in that position. He will resume again by doing his second or first attack.
Eternal Daddy Long Legs acts similarly to its normal counterpart, but his stomp will always release additional 8-way bullets. He also spawns 5 spiders each time.
Completing the Womb multiple times unlocks Triachnid as an alternative boss to Daddy Long Legs.
From the pre-release image of Daddy Long Legs, while Wrath of the Lamb was in development, Daddy Long Legs's head sports a pair of hands and both of his back legs are bent. However, when the DLC was released, Daddy Long Legs' hands were removed while his back legs are straighten.
Daddy Long Legs is a reference to the arachnids known as Opiliones, which are commonly called daddy long-legs, daddy long-legger, or even granddaddy long-legs.
Daddy Long Legs can "boot" Isaac out of a room with his stomp attack if Isaac is standing close to a door, it will do this regardless of the door being closed, the room being an unexplored room, or while in The Chest.
The attack where Daddy Long Legs pulls up his head and stomps one foot at a time may be referencing Beady Long Legs from the Nintendo games Pikmin and Pikmin 2.
Unlike most bosses, Daddy Long Legs doesn't have a death animation. Instead, when he dies, blood pours from the walls, resembling Blood Rights.
Edmund McMillen confirms that Daddy Long Legs is not Isaac's dad's monster form.
Daddy Long Legs is missing his jaw on his intro screen, just like the head of the Headless Horseman, though it does appear he has one when he summons enemies.
In Rebirth, Daddy Long Legs appears with 3 limbs rather than 4. It was probably an error made by the developers. Despite this, he still possesses 4 legs while attacking.
Battling Daddy Long Legs.
Daddy Long Legs trying to stomp the player with his foot.
Daddy Long Legs' head flattened to the floor.
Daddy Long Leg in Isaac's Last Will.
The Binding of Isaac Wrath of the Lamb Bosses Daddy Long Legs (Blind)