The High Luminosity Large Hadron Collider is scheduled to begin in 2027 where faster and more radiation hard detectors are needed to cope with higher track multiplicity and higher radiation levels. With a luminosity of 7.5$\times$10$^{34}$cm$^{-2}$s$^{-1}$ the average pile-up will be about 200 in the High Luminosity Large Hadron Collider, resulting in 1.8 vertices/mm on average. A powerful new way to address this challenge is to exploit the time spread of the interactions to distinguish between collisions occurring very close in space but well separated in time. In this context, timing information can be used to resolve the ambiguities. In a high pile-up event, multiple vertices will occur in the same $z$ but, by adding timing information, vertices could be distinguished. A High-Granularity Timing Detector, based on low-gain avalanche detector technology, is proposed for the ATLAS Phase-II upgrade to mitigate pile-up effects and improve the overall ATLAS performance in the forward region by combining the High-Granularity Timing Detector high-precision time measurement with the Inner Tracker position information. The High-Granularity Timing Detector layout, including sensors and readout electronics are presented. The test beam results are discussed as well.