This page of the guide to Elden Ring explains what causes the tombstone icon to appear on the left side of the screen and what does it signify. You'll also learn how to obtain an item reacting to the tombstone icon.
What does the tombstone icon mean?
The tombstone icon appearing to the left of the screen means you can use the ashes to summon reinforcements. It depends on what kind of summoning ashes you have and how much mana you have, as you must spend it to summon someone.
How to summon?
To unlock the possibility of summoning spirits you must head to the Church of Elleh after obtaining a whistle that summons your mount, then rest at a Site of Grace and choose night as the time of day. Renna the Witch will appear - you must talk to her and confirm that you can summon a mount. She'll reward you with a bell used to summon spirits.
When does the tombstone icon appear?
Even though you already have the bell and some of the spirit ashes, you won't always be able to summon allies. Summons are only available when the tombstone icon appears on the screen.
It usually appears in areas that are problematic to get through alone. These are usually camps, locations with powerful enemies or boss arenas. The appearance of a tombstone icon means that a serious battle awaits nearby.