What's the meaning of " leave him open to the summons " in The Lord Of The Rings?

Question

From book 1|Chapter 2 of The Fellowship of the Ring by J. R. R. Tolkien

The Ring of the Enemy would leave its marks,too, leave him open to the summons.

What is the meaning of the phrase in bold?

Answer 1

The context of this sentence is Gandalf’s description of the search for Gollum:

‘But I am afraid there is no possible doubt: he had made his slow, sneaking way, step by step, mile by mile, south, down at last to the Land of Mordor. […] Alas! Mordor draws all wicked things, and the Dark Power was bending all its will to gather them there. The Ring of the Enemy would leave its mark, too, leave him open to the summons.’

_{J. R. R. Tolkien (1954). The Fellowship of the Ring, book I chapter 2. London: George Allen & Unwin.}

So the last sentence describes the effects on Gollum of his long possession of the Ring. To ‘leave a mark’ means ‘to make a permanent impression’ (OED): that is, Gollum has been permanently changed by the Ring, so that even though he lost it to Bilbo many years ago, he still suffers from its influence. To be ‘open to’ something means to be ‘attentive’ or ‘receptive’ to it: that is, Gollum is still receptive to the magical ‘summons’ of the ‘Dark Power’ or ‘Enemy’ (that is, Sauron, whose very name Gandalf prefers to avoid).

What we have learned in this chapter is that the rings of power weaken and corrupt the possessor until they are under Sauron’s control:

Nine [rings] he gave to Mortal Men, proud and great, and so ensnared them. Long ago they fell under the dominion of the One, and they became Ringwraiths, shadows under his great Shadow, his most terrible servants.

And we have read the inscription on the Ring:

One Ring to rule them all. One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.

So by ‘summons’ we understand the magical power that ‘brings them all’ (that is, all the ring-bearers) to Mordor and ‘binds them’ there (that is, subjects them to the control of Sauron).