The ancient sigil is an object used to access the chamber that Telos inhabits. It is made from combining the Seren, Sliske, Zamorak, and Zaros sigil pieces together. They are obtained by killing the four generals within the Heart of Gielinor.
Assembling the sigil pieces results in the message As you interlock the pieces the edges appear to melt away, binding the parts to create an ancient sigil.
Afterwards, the sigil has a Look-at option which results in the message An ancient sigil adorned in markings that can be seen around the Heart. The edges look like they might lock into something.
The Ancient sigil is used one time to open the door found north of the Heart's truce area. When used, it will give the message The sigil fits perfectly into an impression, becoming one with the door and releasing the seal. It may now be used to venture further into the Heart. After this, the sigil is consumed, and the door remains permanently open.