Nexus Obelisk

The Nexus Obelisk is a special type of turret. It is located at each team's spawning pool and has no health, as it cannot be targeted or damaged. On Summoner's Rift, Proving Grounds, Twisted Treeline, and Howling Abyss, the Nexus Obelisk is near the Shop on each team. It deals 1000 true damage per shot with an attack speed of 2.0. Its damage is not blocked by any type of defensive abilities or stats such as armor, magic resistance. On the Crystal Scar, there are 3 Nexus Obelisks on each team: 2 located on the base entrance, and one near the shop. These Nexus Obelisks can be blocked by abilities that the Obelisks on the three other maps go through.

Abilities and items ignored
Champions that have any of these and walk into range of the Obelisk will be unaffected by any of those effects and will be damaged as normal, except in Dominion and Howling Abbys.
 * All Shields
 * All Shields
 * All Shields
 * All Shields
 * All Shields
 * All Shields
 * All Shields
 * All Shields

A champion that is revived by  will have another death if they are targeted by the Obelisk too.

Abilities and items unaffected
Using these abilities and/or item at the right time within the Obelisk's range will prevent damage for that given time.

Strategy
The obelisk can be used as a weapon when defending the Nexus if the enemies are close or near range of the Obelisk. Champions with crowd control abilities that are able to change the position of an enemy can be used to force them into range of the Obelisk, quickly melting down their health or even killing them. Baiting enemies into the Obelisk's range (melee is easier) can also bring the same result.

and  in particular can grab and hold an enemy in range of the obelisk.